GENERAL GEOLOGY.

Though the geology of the region presents considerable complexity of detail, the general historical facts are distinct and go to show that the whole complex of formations and systems is capable of separation into four major divisions widely different from one another. The mutual relationships of these divisions, a knowledge of which is essential to a thorough comprehension of the geology, are succinctly expressed by the accompanying diagram.

Fig. 3.—Diagram illustrating geological relationships of Montreal River district.

The oldest division, the Keewatin, comprises a complex association of metamorphosed rocks, principally eruptive, characterized by well-developed, secondary schistosity and prevalently dark colours. They dip at angles approaching 90° and range in texture from soft, fissile, chorite schists to fine-grained gneisses or altered diabases. In the Montreal River district the Keewatin areas are not entirely visible, being overlain by other rocks, but they are thought to represent the bottoms of trough-like folds, produced by the upward intrusion of igneous matter which now constitutes the Laurentian. The latter forms the second division, its origin being apparent from the foregoing statement. It is wholly igneous, consisting of granite and allied coarsely crystalline rocks essentially pale-coloured owing to their richness in quartz and feldspars. Gneissic structure has been developed in varying degrees, so that all gradations between granite and gneisses exist; but it never attains the perfection found in theKeewatin. Near their contacts with the Keewatin, the gneisses are apt to contain dark bands and ribbons of the latter so highly crystalline as to conceal their identity.

Wherever visible the surface of the Keewatin and Laurentian presents an irregular, deeply worn appearance, the result of extremely protracted exposure to erosive agencies. To the best of geological knowledge the same conditions hold where they lie buried under the Huronian, indicating that a great period of denudation separates the latter from the Archæan. The combined Keewatin and Laurentian, or Archæan system, is therefore to be conceived as forming at all points in the district an ancient denuded foundation or floor upon which rests the much younger Huronian system.

This third division is, in the Montreal River district, wholly sedimentary and easily distinguished from the other rocks by its bedded structure and clastic nature. As it is the only sedimentary system represented, its members are not easily confused with any other, especially as their original structure is not obscured by metamorphic alteration. Locally this is not strictly true; in the vicinity of diabase intrusions they have been hardened and shattered so as to simulate the Keewatin, but the zones of alteration are narrow and readily identified by their gradation into adjacent areas of less altered types. At present the Huronian forms a discontinuous rock mantle over the Archæan, formerly more complete, but now worn through in places so as to expose portions of the crystalline basement.

The fourth division includes all eruptives known to be younger than the Huronian. Owing to the discontinuity of the latter it is not always easy to decide what rocks should be included in this group, for in some cases rather fresh-looking eruptives occur in the Keewatin which probably would also intrude the Huronian were it present; lacking the necessary information their chronological position can be only loosely fixed. By far the most extensive and important of the post-Huronian eruptives is the diabase with which the silver deposits are associated. This penetrates both Archæan and Huronian, but is ordinarily distinguishable by its unusual freshness, dark colour, and crystalline appearance. In certain cases, to be described subsequently, it may be confused with certain other diabases. Magmatic differentiative processes have evolved diabase types of very dissimilar appearance and mineralogical composition, of whicha pink aplite occurring in dike form is the most extreme. Olivine diabase dikes are also present in the region, but in far less abundance.

Of little importance are the sands and gravels of glacial origin which lie thinly in the depressions and lower lands of the present glaciated surface.

The geological events may be briefly enumerated in ascending order as follows:—

Deposits of glacial débris and weathering products of present surface.

Erosive period with glaciation.

Diabase intrusions.

Huronian sedimentation.

Erosion period.

Laurentian intrusion.

Keewatin.

This system is considered as a complex assemblage of metamorphosed igneous rocks whose common and marked characteristics are pronounced alteration and deformation, accomplished in pre-Huronian times. A limited amount of sedimentary material, such as the iron ore formation, is also represented. These old diabases, porphyries and related types are much altered and have developed a more or less uniform schistosity through the secondary development of micaceous minerals, but in other respects the complex shows extreme inconstancy and variety from point to point. For this reason the various localities are separately described. By inspection of the map three fairly well defined areas are distinguishable, in addition to which are portions of several others.

Much of the country between Obushkong and Firth lakes is underlain by Keewatin. On the east, south and west sides, these rocks disappear beneath the Huronian or are interrupted by masses of diabase, but on the north they merge into Laurentian gneiss, the contact with which is ill-defined. Although some of them retainmuch of their original massive character, well defined schistosity is the dominant feature. The schists stand vertically or at angles little less than 90° and trend in a general east and west direction. A series of compass observations made at points over the whole area show the strike to vary from N. 65° E. to S. 75° E.

One of the most abundant rock types is a stratiform, finely speckled hornblende gneiss or schist, the black hornblende cleavage faces giving it a glistening appearance on newly broken surfaces. It is quite fresh, perfectly crystalline and usually eminently fissile, but sometimes grades into a nearly massive dioritic form of undoubted igneous nature. Extensive exposures occur around Gould lake where the gneiss is traversed by numerous stringers of quartz, rusty in colour from the oxidation of pyrite. It is also well exposed near McLaughlin and McIntosh lakes and to the east and south-east of Foot lake. A fine grained chlorite schist of dull greenish black colour is common in this and all the other areas in the district. To the north-east of Serpentine lake it appears as a sheared phase of a weathered diabase, but it has also been derived from porphyry, exposures being seen between Foot and Obushkong lakes, where feldspar phenocrysts appear on weathered surfaces of the schist as pale, oblong spots. What is probably iron formation was observed at points 25 chains south of Gould lake, and 10 chains south of a little pond just east of Serpentine lake. Both outcrops consist of banded, grey quartzite interlaminated with chlorite schist, but magnetite-bearing bands were not found. Occasionally, narrow bands of pale grey, felsitic schists may be seen among the more common darker rocks. From evidence obtained at various points, these appear to have resulted from the decomposition and shearing of granite porphyry dikes probably connected with the Laurentian, and which penetrated the Keewatin during the time of Laurentian intrusion. Serpentine was observed between Foot lake and Obushkong at 20 chains from the latter. The surface is covered by a loose network of fine seams of asbestos which weather white and render the rock somewhat conspicuous. Its recognition is further simplified by the dull green, amorphous appearance of fresh surfaces, the slight translucence of thin edges and the glistening green seams of asbestos which traverse it abundantly. The same rock is more extensively exposed on the south-west of Serpentine lake and on Firth lake halfa mile north of the portage leading to the former, also at less than a quarter of a mile south of this portage. It is associated with and derived from a dark green massive rock to which the name wehrlite is applicable, and a more detailed description of which appears later.

A Keewatin area of considerable extent lies between Duncan lake and the West branch in the vicinity of L’Africain and Beaverhouse lakes. Unlike the Obushkong area, the prevalent strike of the schists is nearly north and south, the greatest divergence noted being 25° W. In the former case Laurentian lies to the north, while in the present one it occurs on the east; in both cases the schistosity coincides approximately with the direction of the line of contact. On account of the swampy character of the country just west of the river and the scarcity of outcrops, this area was not completely explored, but wherever examined the Keewatin, as in the Obushkong area, consists predominantly of hornblende and chlorite schists, greenstone and decomposed diabase; but serpentine was not found. A nearly black diabase containing small grains of pyrite was observed 20 chains south-east of the southerly extending bay on Duncan lake, and outcrops of the same material were traced for about half a mile northward. Probably the same type was encountered just north of Beaverhouse lake and at some other points. Its unfoliated condition and fresher appearance than the adjacent rocks lead to the opinion that it is really post-Keewatin and intrusive, but the absence of younger rocks with which to correlate it, renders its exact age indeterminable. This rock exhibits enough resemblance to fractured contact edges of the post-Huronian diabase to make their distinction in the field rather difficult. At half a mile east of the south end of L’Africain lake is diabase which from lack of sufficient data has been mapped as Keewatin, although it may be identical with the post-Huronian variety. L’Africain lake lies in glistening hornblende gneiss with which are associated ribbons of a coarser hornblende gneiss belonging to the Laurentian of the area farther east. Greenstone and chlorite schists are the principal rocks around Beaverhouse lake. Associated with them and to the northward, are dikes of light coloured, granite porphyry, usually only a few feet wide, in some of which the original massive structure remains, while others show various gradations toward a felsitic, sericite-bearing schist. The isolated patch ofKeewatin indicated on the map as occurring about half a mile east of the wide portion of Duncan lake, is a well foliated green schist in which oval white spots represent squeezed feldspar phenocrysts.

Fig. 4. View looking north from middle of Duncan Lake, 550ʹ hill in distance.

West of Pigeon lake and the Montreal river, much of the country examined is underlain by Keewatin. Schistosity is less developed than in either of the preceding areas. The rocks are chiefly diabases and porphyry much decomposed and locally squeezed. Diabase occurs on both sides of Pigeon lake near its central islanded part and along the bay which extends south to Brush lake. An original diabase structure is sufficiently well preserved to show lath-like feldspars in hand specimens, but frequently the rock is altered to a chloritic mass. Around the bay extending toward Brush lake, this old diabase forms a rude wall intersected by a reticulating system of fractures filled with calcite, the resultant structure simulating that of a breccia. Shear zones seen on the east shore exhibit a pseudo-conglomeratic structure, the more resistant pieces of diabase having been partially rounded by the shearing movement and embedded in a matrix of finely pulverized rock matter. The same diabase apparently occurs all along the Montreal river where it follows the Algoma-Nipissing line. About half a mile above Pigeon lake it forms a 90 foot cliff on the west side of the river, near the top of which is a fresher looking, unfoliated eruptive. The form of this body was not ascertained, but microscopic examination of the specimen taken, determines it to be a hornblende lamprophyre, so probably it is a dike or thin sheet. The rock is fine-grained, dark green in colour and characterized by stout prisms of hornblende about one-quarter of an inch long, embedded in a finer, microcrystalline ground mass. Under the microscope it appears much decomposed. This rock was also observed near mile post 67 of the Algoma-Nipissing boundary, near the east end of the 63 chain portage leading to Breese lake and at other points, in all cases the exposures being of small extent.

In the last mentioned locality the predominant rock is an altered porphyry of ash grey colour. Around Porphyry lake it occurs in a fairly massive, easily recognizable condition, the feldspars showing as square white spots one-quarter of an inch in diameter in a grey, ground mass. On Breese lake it has been squeezed to a felsite schist, striking N. 20° W. In small amounts it is associated with post-Huroniandiabase on the hills lying near the Montreal river and just south of the 63 chain portage.

The 300 foot hill south-west of Mosher lake is composed of a fine Keewatin greenstone whose surface is curiously weathered so as to suggest a spheroidal structure. It is marked off into round areas a foot or more in diameter by a sinuous network of weathered-out grooves. The main body of the rock is ordinary fine-grained greenstone, but the enclosing grooves have been formed in porphyritic zones about an inch in width. Among other types of less abundant distribution is an actinolite rock seen by the unassisted eye to consist of a felty mass of acicular crystals of actinolite, sometimes half an inch long. This rock was observed 12 chains east of the islanded part of Pigeon lake; also at points west of Montreal river below Pigeon lake.

The Keewatin formation occupies the space between Near lake and the West branch, only part of which has been mapped. The rocks of this area are well foliated, standing as usual, vertically, and striking about N. 60° E. The exposures on the east shore of Near lake are entirely of fissile, chlorite schist, abundantly traversed by small barren quartz veins. Farther east the chlorite schist gives place to glistening, stratiform (laminated) hornblende gneiss with which are associated bands of a coarser hornblende gneiss, evidently the equivalent of granite. In one locality the stratiform rock contains subangular fragments of the coarser variety, quartz and a greyish eruptive rock, the resultant structure resembling that of a metamorphosed conglomerate or breccia. As some of the brecciated fragments occur near by in continuous bands within the stratiform gneiss, the clastic structure is to be ascribed to deformative movement instead of original sedimentation. A number of fine-grained quartz diabase dikes occur in the immediate vicinity.

A considerable portion of the east shore of Kenisheong lake consists of high, bare Keewatin hills, composed largely of chloritic schists, vertically inclined and striking east and west. Secondary calcite is richly disseminated, causing free effervescence when the rock is treated with acid. Pyrite is also abundant, sometimes segregated to form a lean ore, and at several points weathered superficially to limonite, colouring the cliffs dull red. Much-altered diabase like that on Pigeon lake is also present in subordinate amount.

Fig. 5. West Branch Montreal River, near Mosher Lake.

Keewatin rocks also occur just east of Gowganda lake. A single brief visit was paid for the purpose of observing the iron formation which appears at this place, and further mention of which is made on a subsequent page.

Practically all the Laurentian lies in a continuous area, east of the West branch and extending beyond the limits of the map sheet. A smaller body occurs just west of Pigeon lake. The foliation in the Laurentian is less distinct than in the Keewatin and much less perfect. Its component rocks are mainly granite and allied plutonic types which pass by easy gradations into well-defined gneisses. Two principal granites are distinguishable; one containing black mica as its chief coloured constituent, the other hornblende. Both exhibit local variations in composition and crystalline texture, but are always pale grey rocks of ordinary granitic appearance.

The hornblende granite is a medium grained, fresh looking rock of speckled appearance, owing to the black hornblende crystals which lie scattered through the main mass of light grey feldspar and quartz. It is of uniform aspect, local variations of colour and texture being insignificant. As revealed under the microscope by a single thin section, it is an ordinary hornblende granite verging towards a syenite. Common green hornblende of idiomorphic prismatic habit, and usually twinned parallel to 100, is the principal ferromagnesian mineral. It is quite fresh, hence a few flakes of chlorite in the section were taken to represent an original small content of biotite; an acid oligoclase and orthoclase are the most abundant constituents; quartz is subordinate. Apatite, zircon and iron ore, probably magnetite, are accessory.

Hornblende granite is the commonest Laurentian rock in the neighbourhood of L’Africain and Sedge lakes, where it is in contact with the Keewatin. It also occurs on the East branch just below Obushkong lake. The area west of Pigeon lake seems to be composed wholly of this rock, outliers of which extend to Brush lake, and the islands and east shore of Pigeon lake. Here, however, it is distinctly syenitic, quartz being subordinate or absent. A distinctly porphyritic structure is apparent on Pigeon lake; the feldspars beingwell crystallized and lying in a finer grained, holocrystalline ground mass.

Biotite granite and gneiss are most prevalent in the northern and eastern portions of the area. No microscopic examination of these rocks has been made by the writer, consequently, little can be said regarding their composition. They are of much the same texture as the hornblende type, but show a somewhat higher degree of gneissification. Biotite is a fairly abundant constituent, but is more or less altered to chlorite. Sometimes, as may be seen on the East branch, two miles below Obushkong lake, chloritized mica forms enclosing films about the large feldspar grains, developing a slight ‘augen’ structure. Stockwork-like quartz veins are frequent, particularly on the West branch between Near and Sedge lakes and at the south end of Kenisheong lake. At the latter point the biotite gneiss is well foliated and steeply tilted. A body of deep red, biotite granite of undetermined extent and relationship was observed on the east side of Kenisheong lake. Pegmatite dikes almost certainly occur in this region, although not actually observed. Near Crotch lake the granite is locally of unusually coarse texture, although not truly pegmatitic.

Although the two granites described appear to be the essential constituents of the Laurentian they usually contain a variable proportion of other material, some of which at least is not really Laurentian. Over the entire area, but more noticeably in the vicinity of contacts with the Keewatin, they include narrow ribbons or lenses of a glistening stratiform hornblende gneiss, closely resembling the gneiss of this sort described under the Keewatin system. In some cases these bands are portions of the Keewatin caught up by the Laurentian material at the time of its intrusion and highly metamorphosed by it. Excellent examples of this may be seen on the East branch at the foot of the marshy stretch two miles below Obushkong lake. These inclusions are mapped as Laurentian, being an almost constant feature, and for map purposes inseparable from it. They are to be distinguished—a difficult matter—from other dark inclusions believed to be drawn out, basic segregations of Laurentian magmas; the latter are commonly less sharply defined.

Like the other formations of the region, the Laurentian is cut by diabase dikes of post-Huronian age which, owing to their dark colour, are conspicuous among the lighter granite rocks. In the neighbourhood of Zigzag lake they are abundant, and are usually from twentyto sixty feet or more in width, the smaller of which are not large enough to map.

The relationships of the Laurentian to the other systems of the region are expressed by the contacts with them. Between Obushkong and Firth lakes and toward L’Africain lake it is largely in contact with Keewatin. This contact is a vaguely defined zone rather than a line, the formations being separated by the intermediate strip containing mingled portions of both. Proceeding across this strip from the Laurentian to the Keewatin, the gneisses of the former become charged with ribbons of highly crystalline schist, already described. Near the Keewatin edge these increase in quantity and sometimes appear as tongue-like protrusions of that system. This condition exists along the east of L’Africain lake; on the shores of the lake the formation is dominantly Keewatin, but the stratiform hornblende gneiss is traversed along the strike by thin bands of Laurentian hornblende gneiss. A few chains eastward the gneiss bands are wider, and, at a distance of 10 chains, hornblende granite is continuous.

Where terminated by diabase the contact is definite and not marked by notable alteration on either side. Contacts with the Huronian are equally sharp and unconformable, in every case the Laurentian disappearing beneath the sedimentary formation; at the south end of Kenisheong lake the Huronian has been trenched to a depth sufficient to expose the underlying gneissic floor near the water’s edge.

Much of the area mapped, especially the north-western portion, is composed of Huronian rocks. Originally they must have been much more extensive, probably continuous, but erosion has removed them partially or entirely, leaving irregular remnants distributed over the Archæan. The top of the series is gone; consequently complete vertical sections cannot be found. In other respects they are little changed and preserve almost perfectly their bedding and clastic structure. Even their positions assumed at the time of deposition have changed little for the present strata seldom dip more steeply than 30°. They are overlain only by unconsolidated Pleistocene materials.

Basal Conglomerate.

The basal member of this system is a conglomerate, the areal exposure of which is inconsiderable for the whole region or for any part, but erosional sculpturing has brought it to light at many points. Both top and bottom and probably all intermediate portions are visible, but a complete section from which to determine the thickness could not be found. The greatest continuous vertical section observed, in a hill lying one mile south of the 550 foot hill north-west of Duncan lake, is about 200 feet, but neither upper nor lower terminations were seen. In the neighbouring Cobalt district an estimated thickness of 500 feet has been assigned.

From a study of the pebbles contained, this conglomerate appears to be composed largely, or entirely, of Archæan materials, the majority of which are rock types occurring in situ at many places in the district. Both the hornblende and biotite granite and their gneissic phases are abundant. Pegmatite pebbles are sometimes found, but more usually that rock is represented by fragments of feldspar. Pieces of glistening stratiform hornblende gneiss and similar stratiform mica gneiss or schist, greenstone, fine-grained porphyroids and sheared basic rocks represent the Keewatin. Vein quartz is also present, sometimes mineralized. In addition to these some pebbles were observed which closely resembled slate, and one which is undoubtedly conglomeratic. A thin section of this pebble exhibits distinct clastic structure, and the assembled fragments are heterogeneous, so far as distinguishable, including an actinolite rock resembling that found in the Keewatin near Pigeon lake, and a quartzitic type composed of a colourless, microcrystalline mosaic. The cement has been altered largely to chlorite, but small grains of quartz are discernible in it. It would seem, therefore, that sedimentary deposits antedate the basal conglomerate and supply a portion of its materials; most of these, however, are recognizable as igneous members of the Archæan, such as occur in the vicinity.

The pebbles of the conglomerate exhibit remarkable variety in form, size and abundance, as well as composition. Normally they are well rounded and owe their form evidently to water action. But along with these are others which are angular or sub-angular. The exposure near the north end of Shallow lake shows all these forms, the materials also being quite diverse. Ordinarily the pebbles range in size between 2ʺ and 3ʺ diameter, frequently more, and areabundant; but, at points on Duncan lake, on Wapus creek and the West branch, the conglomerate nature is indicated only by occasional well-rounded pebbles, embedded in greywacke cement at intervals of several feet or even yards. These isolated pebbles are often 6ʺ or more in diameter, and in striking contrast with the uniformly fine grain of the matrix. Sometimes the enclosed bodies attain the dimensions of boulders; on an island in Duncan lake,21⁄2miles from the foot, a granite boulder nearly 5 feet in diameter was observed, the associated materials being of very much smaller size.

The cementing material also presents some variety. Usually it ranges from coarse grit to greywacke. The coarser material is less abundant than cement of a finer type; near the foot of Pigeon lake, also at the south end of Kenisheong lake the conglomerate is quite porous, the interstices between the pebbles being incompletely filled. In the latter instance these spaces are occupied by hematite. In other cases; a good example of which occurs on the east side of Pigeon lake, half a mile from its north end, the cementing substance is a fine black shale. On the whole there appears to be little relationship between the texture of enclosing and enclosed matter.

By the disappearance of pebbles the basal conglomerate changes into a greywacke or a shale according as the cement is one or the other. Frequently this transition is marked by an alternation of lenticular beds of conglomerate with the greywacke, probably indicating varying conditions of depth or current in the water in which they were deposited. With the greywacke and shale is associated an impure quartzite or arkose, the whole forming a thick series whose members are not sharply separable one from another and do not occupy definite relative positions. For the well laminated finer grained beds the term slate is in general use, although a secondary cleavage by which this kind of rock is distinguished from shale does not exist. Nearly all the prominent hills in the north-western part of the area are composed of this series.

The greywacke-slate-quartzite series passes conformably upward into a conglomerate differing little from that at the base. Indeed, where conglomerate outcrops are small and isolated a distinction between the two cannot be made with certainty.

Arkose.

In addition to the above there is an arkose forming at least two well defined areas, whose relations with the rest of the Huronian are in some doubt. This rock is of distinctive appearance, resembling at first glance an ordinary granite, but on closer inspection it is seen to consist of clastic materials such as would result from the disintegration of a granite. Occasional conglomerate streaks in which pebbles of quartz and greenstone are recognizable, demonstrate its bedded character, but ordinarily it is massive looking. Part of the large island in the middle of Duncan lake and the shore to the south are of this formation. It is much more widespread on Obushkong and Gowganda lakes, the bedding being unusually well shown on the large island in the latter body of water.

From its apparent relationship in either of these localities it might be taken to be a member of the Lower Huronian series just described, and equivalent to the basal conglomerate. In the Obushkong area it is probably underlain by Laurentian, for it lies nearly horizontally, and Laurentian is known to occur a short distance east of the lake. In the neighbourhood of Lake Timiskaming it is said to grade imperceptibly into granite, and is believed to be derived from the latter by detrition in situ. Arkose, apparently identical with that under consideration, occurs in the Cobalt district, and is thought to lie unconformably with the greywacke, and for this reason is classified as Middle Huronian. In these pages it is given no definite position in the formational succession, and as there is no field evidence of its Middle Huronian position, is not differentiated from the lower series.

The Huronian has been subjected to no very severe disturbance, judging by its present condition and attitude. Frequently the strata lie almost horizontally, as for instance at the south end of Firth lake, and in the vicinity of Lake Lehmann, but over the most of the region they rest at inclinations as high as 30°, this attitude remaining constant over extensive areas, and developing a characteristic topographical feature. From Pigeon lake eastward the dip is uniformly to the east at angles ranging between 15 and 30 degrees and the beds overlap one another after the fashion of slates on a roof, the resultant topographical expression of which is a succession of northand south ridges with gentle eastern slopes, while the western sides form escarpments. This condition appears constant over all the Huronian east of a line midway between Pigeon and Duncan lakes. To the extreme north-west, however, a westerly dip was observed; the strata forming the 550 foot hill west of Duncan lake are also either horizontal or dip gently to the west. A confident statement cannot be made until further work has been performed, but the condition just outlined suggests a large anticlinal structure whose arch lies a little west of Duncan lake. However, there seems reason to believe that the structure is more complicated than would result from simple arching and erosion. If the present overlapping system represents the original succession of strata a total thickness of over two miles would be necessary, and there would be not two but several conglomerate horizons, which is improbable. A satisfactory solution is hindered by the general fact that planes of possible dislocation occur in low ground and are obscured by swamp or water, but it seems evident that tilting was accompanied by lateral or vertical displacements.

Certain abrupt disturbances of the general uniform attitude suggest differential movements. Along the West branch below Wapus creek the dip and strike of the shale and greywacke are constant, until where the river’s course changes to due east. Along the shores of this stretch the rocks are mostly hidden by swamp, but where they do outcrop they are standing vertically or dipping steeply to the south, and the strike corresponds with the course of the stream; that is, their positions are at right angles to those farther south. Disturbances of this sort are known, due to the contiguity of igneous intrusions, but at this point no such intrusive is known. It will also be seen from the map that exactly in the same line the East branch makes a similar abrupt change in course. Whether a line of low relief is continuous across the interval between the two streams at this point is not easily determined; however, as a possible explanation of the conditions stated, faulting along this line is suggested. A similar abrupt change from conditions of approximate horizontality to a dip S. 60 W., < 80° and strike of S. 30° E. was noted on the west side of Firth lake.

The intrusion of the quartz diabase into the Huronian was gently accomplished at most points, and the beds of the latter, both aboveand below the intrusives, are inclined only a few degrees more steeply than in localities where no diabase can be found. Evidence of intrusion, however, is common, and at some points the Huronian next to the diabase has suffered local but intense physical change. Near the middle of the east shore of Firth lake a rocky islet only a few square yards in extent consists of coarse diabase and conglomerate in intimate contact, little tongues of the former being protruded into the sedimentary rock and peripherally chilled. Most remarkable, however, is the change in condition of the conglomerate; a few chains away on the main shore it is of ordinary character, but on the islet the pebbles lie within a fairly well foliated schist, standing vertically and striking about east and west. An identical condition exists at the south end of a little pond lying 40 chains west of Mosher lake. The vertical foliation of the conglomerate suggests the neighbouring diabase intrusion to have been by vertical ascension rather than lateral spread, so that these points may represent portions of vents through which the diabase magma ascended, and for that reason are more affected than where sills have been quietly injected.

Ordinarily these contacts occur in low ground, usually ravines, the bottoms of which are soil filled and consequently unfavourable for geological observation, but at some points on Duncan lake the contacts are exposed and the Huronian is seen to be much fractured across a zone extending many feet from the diabase. In addition to the fracturing the greywacke is hardened and the bedding planes rendered obscure, the total effect being to weaken its power of resistance to erosion.

In the neighbourhood of these contacts the Huronian is well supplied with quartz veins whose ‘comb’ structure and chalcopyrite-galena mineralization identify them with similar veins in the diabase. Also the sediments exhibit certain mineralogical alterations referable to action of the diabase. West of Gowganda, on the West branch below Duncan lake and at other places the greywacke for a width of about two feet from the diabase has been hardened, bleached a light grey colour and filled with circular black spots1⁄16ʺin diameter. Microscopic examination shows the main portion to consist of a mosaic of small quartz, orthoclase and acid plagioclase grains, through which are distributed patches of chlorite. This spotted phase of the greywacke evidently represents an early stagein the development of adinole, a characteristic contact product of shales, intruded by basic rock.

Fig. 6. Fifth Portage on the West Branch of Montreal River, showing Huronian Slate.

The relations of the Huronian to the Archæan, exhibited in larger as well as minor features, indicate that the surface of the latter had been carved into a condition not greatly unlike the present topography, before its submergence and sedimentation in Huronian times. Some of the hills of this ancient land have been uncovered by the removal of their sedimentary blanket, leaving vestiges of the latter around their bases as in the case of the hills south-west of Mosher lake. There the dip of the sedimentary beds is much less than the slope of the hills, so that the latter must project up through them as cores. In the bay on the east side of Pigeon lake Huronian shales dipping with an angle of about 15° abut against the side of an Archæan hill composed of hornblende granite and green schist. At almost any part of the region where both Huronian and Archæan occur together, similar evidence is available concerning the unevenness of the pre-Huronian land surface. The amount of topographical relief cannot be estimated with any degree of accuracy, for tops of the Keewatin hills now exposed have probably been removed by post-Huronian erosion, the lowest depressions are still filled by Huronian and the whole may have been disarranged by post-Archæan faulting. However, near Mosher lake there was a minimum relief of 300 feet.

This old pre-Huronian surface is not often accessible for study, the contacts being vertical or hidden by soils and vegetation. At one point, however, at the end of the portion of Wapus creek shown on the map, exceptionally favourable circumstances were encountered. Here glaciation has developed a rounded knoll of mixed Keewatin and Laurentian rocks upon which are tightly fastened a few scale-like vestiges of Huronian conglomerate. A few square feet of the original Archæan surface exposed by chiseling away the Huronian, was found to be much more highly polished than the immediately adjoining, recently glaciated surface which had been exposed to the atmosphere. The protected material seemed to be of about the same freshness as that exposed. This pre-Huronian surface is evidently a water-worn one, or the result of Huronian glaciation, but no characteristic markings were observed upon it.

From the foregoing consideration of the Huronian as seen in the Montreal River region it will be plain that it is composed at all pointsof clastic sedimentary deposits. Fossils have never been found, so the correlation of these rocks in various areas is based upon their lithological similarities and continuity. Upon these grounds the Huronian of the present district is considered to be equivalent to the same formations in the Cobalt, Larder Lake, and other neighbouring districts, known as the Lower Huronian. The succession and physical character of the different formations are essentially identical. It is not necessary to review the facts upon which the decision to so place these formations is based, the evidence being the harmonious results of years of investigation by capable geologists. Accepting their conclusion, and calling the main sedimentary series of the Montreal River district, Lower Huronian, it is of interest to note indication of sedimentary materials still older. The conglomerate pebble found in the basal conglomerate on Pigeon lake must have originated by the destruction of a pre-Huronian conglomerate formation.

It has also been stated that ground for differentiating between a Lower and Middle Huronian in this area has not been obtained, although the similarity of the arkose to a formation in the Cobalt district believed to belong to the Middle Huronian suggests it to be of the same age. It is, therefore, thought desirable to apply to the whole the term Huronian. No clearness or additional truth would be gained by making a distinction between Lower and Middle divisions in this district, and so far as the economic exploitation of the district is concerned the arkose seems to be as much a part of the Lower Huronian as the conglomerate or greywacke.

There are two kinds of diabase known in the district, one of which greatly exceeds the other in extent and economic importance. This disparity is tacitly recognized by the common use of the generic term diabase for the important quartz diabase, the other member of the family being ignored or unknown.

Fig. 7. Inclined Huronian Beds, Duncan Lake.

Character of contacts and distribution.—Bodies of this rock are not restricted to any one formation or locality; but, as may be seen by reference to the map, occur with various dimensions over the whole area, with the exception of the country lying north-west ofDuncan lake. It may be noted that virtually all the large areas are enclosed by Huronian, the bodies within the Archæan being numerous but small. It is not proposed to apply this distinction generally, but in the Montreal River region it seems to be more than an expression of the law of probability. In the Archæan practically all of the observed diabase bodies are dikes, in approximately vertical positions, seldom 100 feet in width and of undetermined length. A knowledge of all those in the Huronian could not be obtained, but in some cases they are sills, whose greater exposed dimensions are parallel to the bedding planes of the enclosing sedimentaries.

Macroscopic character.—It is fortunate for the easy recognition of the diabase that it is more or less continuously exposed, and that there are not many other igneous rocks of similar character in its proximity, for it presents a variety of types such that a representative collection of hand specimens presents surprisingly great petrological differences. During the process of solidification, magmatic differentiation evolved from the cooling material a group of forms of different mineralogical composition and physical appearance, the end members of which are very unlike.

The common type is a dark green massive diabase, ranging in texture from one in which the individual minerals are barely distinguishable to others containing amphibole crystals an inch in length. The combination of black amphibole—an alteration product of augite—and grey or flesh-coloured plagioclase give the surface of ordinary textured phases a colouration of sufficient determinative value. In a large diabase body the coarseness is equal to that of ordinary granite, and the diabasic structure is not readily perceptible. At the edge of the large dikes and throughout the smaller ones the rock is black in colour and much finer grained on account of its more rapid solidification.

In the dikes the mineralogical composition is tolerably constant, and specimens selected from different points show no notable difference except that they are usually less decomposed than the coarse grained varieties. Within the larger bodies, where cooling may be considered to have been slower, a series of rock types differing from one another in mineral composition, and consequently in physical appearance, are associated. At many localities these differentiated varieties are intimately intermingled, so thatwithin an area of a fewsquare yards almost the whole series may be found. Conditions of this sort were first and best observed in the Lett properties on Wapus creek, where the extensive stripping and trenching greatly facilitated geological study. A suite of specimens was obtained which exhibit an unbroken gradation from ordinary gabbro to the fine grained pink rock known as aplite. With a decreasing pyroxene content and increasing abundance of feldspar the rock grades from a dark green diabase at the basic end through a reddish phase into a type which, in the field, might be termed a syenite, being of granitic texture, red colour, and without visible quartz. These phases are cut by aplite dikes which at first glance do not very closely resemble them, but their comparison has shown that they too include a group whose coarsest and most basic form does not differ greatly from the syenitic type of the diabase series. From a pale flesh-coloured rock of fine granitic texture in which a little dark mineral is visible, the successive phases of the aplite graduate toward a dike material of light pink colour and saccharine texture almost devoid of ferromagnesian constituents.

Microscopic character.—The consanguinity of diabase and aplite is further established by microscopic study. It was intended to make a somewhat complete comparative study; but, at the outset, the materials, although fresh looking in the hand specimens, were found to be much decomposed, sometimes so completely that the original composition could not be satisfactorily inferred, and an outline must suffice therefore until fresher material is secured.

The common gabbro type consists essentially of long prisms of plagioclase embedded in light reddish augite. This ophitic structure, upon which the distinction between diabase and gabbro depends, is well developed, but does not show in hand specimens, so that for field use the distinction is impracticable; ordinarily the term gabbro is applied to the coarse grained, and diabase to the medium and finer grained varieties. The plagioclase of the coarse grained specimens could not be identified, being entirely altered to a coarse saussurite in which the epidote was aggregated into large grains. The augite is almost equally changed to strongly pleochroic hornblende possessing green and blue green pleochroic tints. This hornblende is not a fibrous variety but forms compact individuals, hence in the specimens studied it could not be certain that some of it was not primary. In some cases it is further altered to chlorite. Reaction betweenplagioclase and pyroxene seems to have taken place, for chlorite occurs among the plagioclase decomposition products as well as those of the pyroxene. Next to these, ilmenite is the most abundant constituent, occurring as irregular masses largely altered to leucoxene in which the original ‘gridiron’ structure is distinct. Quartz is present in subordinate amounts usually in micrographic intergrowth with the plagioclase. Small, well defined hexagonal rods of apatite, titanite crystals, and rare zircons are also present. The structure and mineral constitution render the term quartz-diabase appropriate. Finer grained specimens secured from dikes were found to be much fresher than the coarser types and yielded more satisfactory thin sections; the ophitic structure is more pronounced, but the mineral composition is the same. The plagioclase laths of one section were determined optically to be an intermediate labradorite. Small flakes of biotite partially altered to chlorite occur accessorily.

The aplite is also so much weathered that doubt sometimes exists as to its original composition. Thin sections consist very largely of plagioclase and quartz, coloured constituents being small in quantity. The plagioclase is twinned according to albite, pericline, and carlsbad laws, and in one case a baveno twin was observed; optically it behaves as almost pure albite and is decomposed to sericite instead of epidote. A poikilitic arrangement is more or less distinct; the feldspar is also micrographically intergrown with quartz. No orthoclase could be identified in any of the specimens. Quartz does not appear to be much more abundant than in the diabase. The only recognizable ferro-magnesian mineral is a strongly pleochroic reddish brown biotite, which occurs sparingly in small ragged flakes, partly altered to chlorite. Leucoxene representing ilmenite is surprisingly abundant considering the scarcity of iron bearing silicates. Apatite is an abundant accessory mineral, forming small prisms, while zircon crystals are rare. The rock in the specimens examined is remarkably rich in calcite, which in amount ranks next to the plagioclase.

Comparison of sections from specimens intermediate between the aplite and the ordinary diabase brings out some features of the differentiating process. The governing changes are in the proportion of pyroxene, and in the composition of the plagioclase. A specimen somewhat paler in colour than the typical diabase was found to contain considerably less augite and more abundant plagioclase, theother constituents remaining fairly constant. The plagioclase was an acid andesine of the composition (Ab 65 An 35). A still lighter coloured type possessed oligoclase (Ab 72 An 28) and a small amount of blue green hornblende, representing the wholly altered pyroxene. Orthoclase could not be found in any of the sections, so that none of these rocks can be properly called syenite or granite.

Local description.—Diabase is most abundantly exposed in the area between Gowganda lake and the portage route connecting Firth and Elkhorn lakes, where it forms three parallel tongues of approximately equal dimensions. Coarse grained types prevail in which the differentiation of acid phases is pronounced. Aplite dikes up to 10 feet in width, fairly coarse grained and sometimes tolerably rich in dark constituents, are abundant. The rugged surface of this part of the country is largely due to the presence of the diabase, the peculiar arrangement of which has produced a constant system of north and south ridges. The contacts with the Huronian occupy the bottoms of gullies, with the main body of diabase forming high ground. This highly relieved and consequently well exposed surface has greatly facilitated the exploitation of this area, as indeed is the case for the whole district. Terminally each of the three bodies tapers out or forks, but these details were not closely mapped. In the case of the most easterly one the portions reaching the east side of Firth lake are darker and more decomposed than the ordinary diabase, and during the field operations were considered of other character. Laboratory study of the specimens shows them, however, to be basic diabase in which hornblende has completely replaced the pyroxene. In consequence of this misconception the connexions of the exposures on Firth lake were not well worked out, but they almost certainly lie as shown in the map and may form a continuous connexion with the large diabase body farther north. The two areas north-east of Firth lake present much the same appearance as those of the Gowganda district.

The most extensive body is that which coincides in direction with Duncan lake, resembling a great hook, the shank of which is traceable for twelve miles, the crooked end lying about midway between the West branch and Firth lake. Neither termination was found, on account of the swampy or sandy character of the country, so that the present representation may not be complete. Just east of L’Africain lake there are some diabases which may be continuations of it, but owing to their altered appearance they could not be distinguishedwith certainty in the field from similar Keewatin forms. Along Duncan lake it appears to represent the edge of a sill about 300 feet thick lying in the bedding plane of the Huronian sediments which lie both above and beneath it. From the attitude of the latter the sill is believed to dip eastward at an angle of 30 or 40 degrees. The exposed width is usually less than in the Gowganda area and its composition is more homogeneous, but at the wider portions the same intermingling of basic and acid phases and aplite dikes obtains, as for example where it crosses Wapus creek. The full extent of that portion which extends north-westward from Mosher lake is not known. The formation at that point is obscured by gravelly soil; it appears, however, to be unusually wide and well differentiated. An apparently large body of diabase lies between the north part of Duncan lake and the West branch. The mapping of this was left incomplete, so that its total southern extent cannot be given. Its northern part presents nothing unusual, but about Vipond lake a syenitic phase is developed, probably indicating a mingling of differentiated rock varieties as at Gowganda lake or Wapus creek.

The remaining diabase bodies are dikes. Probably these are very numerous and widespread, but this can only be determined by work of much more widespread nature than the economic possibilities of such an investigation warrant at present. They are most frequently seen in the Laurentian, probably not so much because they are more numerous there, but because they are more conspicuous in the granites than in the Keewatin schists and Huronian, from which they differ less in colour. They attain widths of 60, 100 or more feet, in which cases the texture is like that of the large masses, but differentiated types and aplite dikes do not seem to accompany them. The smaller dikes, diminishing to a width of 1 foot, are compact black rocks of microcrystalline habit, but like the gabbroid varieties mineralogically.

Age.—The time of intrusion of these rocks can only be defined as post-Huronian or post Middle Huronian, there being no younger formations with which to correlate them. They are identical with the diabase in other parts of the Montreal River region and of Cobalt. As already noted the larger masses have produced very limited metamorphic changes in the adjoining rocks, developing incipient adinole zones in the Huronian greywackes, besides hardening and fracturing them. Usually the contact edge of the diabase has been deeply eroded along with the adjoining rock to form narrow ravines, but wherever it persists a slight chilling is perceptible. Thefine grained edges are much more apparent in dikes where the mass of hot material being much less was more susceptible to the influence of cold surroundings. The intrusive nature is also demonstrated by angular blocks of Huronian materials enclosed within the diabase, instances of which may be seen on Wapus creek.

At the first rapid above Kenisheong lake the Huronian is cut by a broad dike of different character from those belonging to the quartz-diabase intrusion. It is a remarkably fresh compact rock of dark grey colour and medium texture in which an ophitic structure is prominent, acicular prisms of glassy feldspar penetrating the dark main mass. Its density is 2·991. Under the microscope it is holocrystalline and the constituent minerals occur in only one generation. The constituents as determined by linear measurements were found to be plagioclase, 67·5 per cent; olivine, 14 per cent; augite, 1·5 per cent; iron ore, probably ilmenite, 5 per cent; apatite, 1·8 per cent; biotite, 1·3 per cent; and zircon, 0·03 per cent. The rock is remarkably fresh, even the olivine showing no signs of decomposition. The plagioclase was determined by optical methods to be a labradorite of the composition Ab 1 An 1. It forms laths, twinned according to albite, pericline and carlsbad laws, which penetrate the ferromagnesian minerals. A few large crystals show fine zonary lamellation. Olivine is in idiomorphic or rounded grains, occasionally bordered by a little rim of biotite, possibly a product of reaction during the period of crystallization. The augite is reddish brown in colour and later than either olivine or plagioclase, filling the interstices between the feldspar laths. Irregular masses of black iron ore, probably ilmenite, are scattered throughout the section, in some cases showing good crystal forms. A deep brown strongly pleochroic biotite with a very small optical angle occurs accessorily in small shreds. Apatite is mostly in slender but sometimes stout hexagonal prisms. Minute crystals of zircon are rare.

The rock may be designated an olivine diabase of markedly fresh aspect. It penetrates the Huronian, but judging by its unusually fresh condition it must be comparatively young.

The present glaciated pre-Cambrian surface is scantily covered by unconsolidated glacial sands and gravel which are being collected by natural agencies from the hills and more elevated parts into thedepressions. As a consequence the hills are bare, while the valleys and ravines are soil-filled and support a strong forest growth. Neither sand nor gravel show signs of stratified arrangement. They play an unimportant part in the topographical appearance of the country; the only elevation composed of such materials being a small gravel hill to the east of Porphyry lake. Between that point and the hills near Mosher lake is a considerable extent of flat sandy country, the gentle contour of which contrasts with that of the surrounding hills.

A rather peculiar low apron of sand occupies the very end of the peninsula between Duncan lake and the West branch. It is of small extent, and is probably due to river deposition, the West branch at this point being sluggish and the surface of the sand showing indications of shifting stream beds.

An extensive area of sand lies at the end of Duncan lake, from whence it extends northward for several miles, also westward and south-westward toward the 550 foot hill shown on the map. Seen from any neighbouring hill this plain appears flat in comparison with the ordinary surface of the country. It consists of a fine, yellowish, unstratified sand. The several brooks crossing it are distinguished from those of other parts of the country by their tortuous courses which are constantly changing, and which render them unfit for canoe travel. The shallow character and sandy shores at the north end of Duncan lake have been caused largely by the transportation of sand into it by these brooks.

Glacial boulders are scattered over the whole district.

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