CHAPTER III

Ancient Volcanoes: Proofs of their existence derived from the Nature of the Rocks erupted from the Earth's Interior. A. Materials erupted at the Surface—Extrusive Series. i. Lavas, their General Characters. Volcanic Cycles. ii. Volcanic Agglomerates, Breccias and Tuffs.

The materials brought by volcanic action from the earth's interior have certain common characters which distinguish them from other constituents of the terrestrial crust. Hence the occurrence of these materials on any part of the earth's surface affords convincing proofs of former volcanic eruptions, even where all outward trace of actual volcanoes may have been effaced from the topographical features of the ground.

Volcanic products may be classed in two divisions—1st, Those which have been ejected at the surface of the earth, or the Extrusive series; and 2nd, Those which have been injected into the terrestrial crust at a greater or less distance below the surface, and which are known as the Intrusive series. Extrusive rocks may be further classified in two great groups—(i.) The Lavas, or those which have been poured out in a molten condition at the surface; and (ii.) The Fragmental Materials, including all kinds of pyroclastic detritus discharged from volcanic vents.

Taking first the Extrusive volcanic rocks, we may in the present chapter consider those characters in them which are of most practical value in the investigation of the volcanic phenomena of former geological periods.

The term Lava is a convenient and comprehensive designation for all those volcanic products which have flowed out in a molten condition. They differ from each other in composition and structure, but their variations are comprised within tolerably definite limits.

As regards their composition they are commonly classed in three divisions—1st, The Acid lavas, in which the proportion of silicic acid ranges from a little below 70 per cent upwards; 2nd, The Intermediate lavas, wherein the percentage of silica may vary from 55 to near 70; and 3rd, The Basic lavas, where the acid constituent ranges from 55 per cent downwards. Sometimes the most basic kinds are distinguished as a fourth group under the name of Ultrabasic, in which the percentage of silica may fall below 40.

The structures of lavas, however, furnish their most easily appreciated characteristics. Four of these structures deserve more particular attention: 1st, Cellular, vesicular or pumiceous structure; 2nd, The presence of glass, or some result of the devitrification of an original glass; 3rd, Flow-structure; and 4th, The arrangement of the rocks in sheets or beds, with columnar and other structures.

Fig. 1.—Vesicular structure, Lava from Ascension Island, slightly less than natural size.

1. TheCELLULAR,VESICULAR,SCORIACEOUSorPUMICEOUS STRUCTUREof volcanic rocks (Fig. 1) could only have arisen in molten masses from the expansion of imprisoned vapours or gases, and is thus of crucial importance in deciding the once liquid condition of the rocks which display it. The vesicles may be of microscopic minuteness, but are generally quite visible to the naked eye, and are often large and conspicuous. Sometimes these cavities have been subsequently filled up with calcite, quartz, agate, zeolites or other mineral deposition. As the kernels thus produced are frequently flattened or almond-shaped (amygdales), owing to elongation of the steam-holes by movement of the lava before its consolidation, the rocks containing them are said to beamygdaloidal.

This structure, though eminently characteristic of superficial lavas, isnot always by itself sufficient to distinguish them from the intrusive rocks. Examples will be given in later chapters where dykes, sills and other masses of injected igneous material are conspicuously cellular in some parts. But, in such cases, the cavities are generally comparatively small, usually spherical or approximately so, tolerably uniform in size and distribution, and, especially when they occur in dykes, distributed more particularly along certain lines or bands, sometimes with considerable regularity (see Figs.90,91, and236).

Among the superficial lavas, however, such regularity is rarely to be seen. Now and then, indeed, a lava, which is not on the whole cellular, may be found to have rows of vesicles arranged parallel to its under or upper surface, or it may have acquired a peculiar banded structure from the arrangement of its vesicles in parallel layers along the direction of flow. The last-named peculiarity is widely distributed among the Tertiary lavas of North-Western Europe, and gives to their weathered surfaces a deceptive resemblance to tuffs or other stratified rocks (see Figs.260,310and311). It will be more particularly referred to a few pages further on. In general, however, we may say that the steam-cavities of lavas are quite irregular in size, shape and distribution, sometimes increasing to such relative proportions as to occupy most of the bulk of the rock, and in other places disappearing, so as to leave the lava tolerably compact. When a lava presents an irregularly vesicular character, like that of the slags of an iron-furnace, it is said to beslaggy. When its upper surface is rugged and full of steam-vesicles of all sizes up to large cavernous spaces, it is said to bescoriaceous, and fragments of such a rock ejected from a volcanic vent are spoken of asscoriæ.

Attention to the flattening of the steam-vesicles in cellular lavas, which has just been alluded to as the result of the onward movement of the still molten mass, may show, by the trend and grouping of these elongated cavities, the probable direction of the flow of the lava before it came to rest. Sometimes the vesicles have been drawn out and flattened to such a degree that the rock has acquired in consequence a fissile structure. In other instances, the vesicles have been originally formed as long parallel and even branching tubes, like the burrows of Annelids or the borings ofTeredo. Some remarkable examples of this exceptional structure have been obtained from the Tertiary plateau-basalts of the Western Isles, of which an example is represented inFig. 2.

In many cases the vesicles extend through the whole thickness of a lava. Frequently they may be found most developed towards the top and bottom; the central portion of the sheet being compact, while the top and bottom are rugged, cavernous or scoriaceous.

Though originally the vesicles and cavernous spaces, blown open by the expansion of the vapours dissolved in molten lava, remained empty on the consolidation of the rock, they have generally been subsequently filled up by the deposit within them of mineral substances carried in aqueous solution. The minerals thus introduced are such as might have been derivedfrom the removal of their constituent ingredients by the solvent action of water on the surrounding rock. And as amygdaloids are generally more decayed than the non-vesicular lavas, it has been generally believed that the abstraction of mineral material and its re-deposit within the steam-vesicles have been due to the influence of meteoric water, which at atmospheric temperatures and pressures has slowly percolated from the surface through the cellular lava, long after the latter had consolidated and cooled, and even after volcanic energy at the locality had entirely ceased.

Fig. 2.—Elongation and branching of steam-vesicles in a lava, Kilninian, Isle of Mull, a little less than natural size.

Examples, however, are now accumulating which certainly prove that, in some cases, the vesicles were filled up during the volcanic period. Among the Tertiary basalt-plateaux of the Inner Hebrides, for instance, it can be shown that the lavas were already amygdaloidal before the protrusion of the gabbros and granophyres which mark later stages of the same continuous volcanic history, and even before the outpouring of much of the basalt of these plateaux. Not improbably the mineral secretions were largely due to the influence of hot volcanic vapours during the eruption of the basalts. This subject will be again referred to in the description of the Tertiary volcanic series.

Vesicular structure is more commonly and perfectly developed among the lavas which are basic and intermediate in composition than among those which are acid.

While the existence of a highly vesicular or scoriaceous structure may generally be taken as proof that the rock displaying it flowed out at the surface as a lava, other evidence pointing to the same conclusion may often be gathered from the rocks with which the supposed lava is associated.Where, for example, a scoriaceous lava is covered with stratified deposits which contain pieces of that lava, we may be confident that the rock is an interstratified or contemporaneous sheet. It has been erupted after the deposition of the strata on which it rests, and before that of the strata which cover it and contain pieces of it. In such a case, the geological date of the eruption could be precisely defined. Illustrations of this reasoning will be given inChapter iv., and in the account of the volcanic series of Carboniferous age in Central Scotland, where a basic lava can sometimes be proved to be a true flow and not an intrusive sill by the fact that portions of its upper slaggy surface are enclosed in overlying sandstone, shale or limestone.

2. The presence ofGLASS, or of some result of the devitrification of an original glass, is an indication that the rock which exhibits it has once been in a state of fusion. Even where no trace of the original vitreous condition may remain, stages in its devitrification, that is, in its conversion into a stony or lithoid condition, may be traceable. Thus what are called spherulitic and perlitic structures (which will be immediately described), either visible to the naked eye or only observable with the aid of the microscope, afford evidence of the consolidation and conversion of a glassy into a lithoid substance.

Striking evidence of the former glassy, and therefore molten, condition of many rocks now lithoid is to be gained by the examination of thin slices of them under the microscope. Not only are vestiges of the original glass recognizable, but the whole progress of devitrification may be followed into a crystalline structure. The primitive crystallites or microlites of different minerals may be seen to have grouped themselves together into more or less perfect crystals, while scattered crystals of earlier consolidation have been partially dissolved in and corroded by the molten glass. These and other characteristics of once fused rocks have to a considerable extent been imitated artificially by MM. Fouqué and Michel Lévy, who have fused the constituent minerals in the proper proportions.

Since traces of glass or of its representative devitrified structures are so abundantly discoverable in lavas, we may infer the original condition of most lavas to have been vitreous. Where, for instance, the outer selvages of a basic dyke or sill are coated with a layer of black glass which rapidly passes into a fine-grained crystalline basalt, and then again into a more largely crystalline or doleritic texture in the centre, there can be no hesitation in believing that glassy coating to be due to the sudden chilling and consolidation of the lava injected between the cool rocks that enclose it. The part that solidified first may be regarded as probably representing the condition of the whole body of lava at the time of intrusion. The lithoid or crystalline portion between the two vitreous outer layers shows the condition which the molten rock finally assumed as it cooled more slowly.

Some lavas, such as obsidians and pitchstones, have consolidated in the glassy form. More usually, however, a lithoid structure has been developed, the original glass being only discoverable by the microscope, and often noteven by its aid. Two varieties of devitrification may be observed among lavas, which, though not marked off from each other by any sharp lines, are on the whole distinctive of the two great groups of acid and basic rocks.

Fig. 3.—Microlites of the Pitchstone of Arran (magnified 70 diameters).

(1) Among the acid rocks, what is called the Felsitic type of devitrification is characteristic. Thus, obsidians pass by intermediate stages from a clear transparent or translucent glass into a dull flinty or horny mass. When thin slices of these transitional forms are examined under the microscope, minute hairs and fibres or trichites, which may be observed even in the most perfectly glassy rocks, are seen to increase in number until they entirely take the place of the glass. Microlites of definite minerals may likewise be observed, together with indefinite granules, and the rock finally becomes a rhyolite, felsite or allied variety (Fig. 3).

At the same time it should be observed that, even in the vitreous condition of a lava, definite crystals of an early consolidation were generally already present. Felspars and quartz, usually in large porphyritic forms, may be seen in the glass, often so corroded as to indicate that they were in course of being dissolved in the magma at the time of the cooling and solidification of the mass. In obsidians and pitchstones such relics of an earlier or derived series of crystallized minerals may often be recognized, while in felsites and quartz-porphyries they are equally prominent. Where large dispersed crystals form a prominent characteristic in a rock they give rise to what is termed thePorphyriticstructure.

Fig. 4.—Perlitic structure in Felsitic Glass, Isle of Mull (magnified).

Fig. 5.—Spherulitic structure (magnified).

Accompanying the passage of glass into stone, various structures make their appearance, sometimes distinctly visible to the naked eye, at other times only perceptible with the aid of the microscope. One of these structures, known asPerlitic(Fig. 4), consists in the formation of minute curvedor straight cracks between which the vitreous or felsitic substance, during its contraction in cooling, assumed a finely globular form.

Another structure, termedSpherulitic(Fig. 5), shows the development of globules or spherules which may range from grains of microscopic minuteness up to balls two inches or more in diameter. These not infrequently present a well-formed internal fibrous radiation, which gives a black cross between crossed Nicol prisms. Spherulites are more especially developed along the margins of intrusive rocks, and may be found in dykes, sills and bosses (see Figs.375and377). Where the injected mass is not thick it may be spherulitic to the very centre, as can be seen among the felsitic and granophyric dykes of Skye.

Some felsitic lavas possess a peculiar nodular structure, which was developed during the process of consolidation. So marked does this arrangement sometimes become that the rocks which display it have actually been mistaken for conglomerates. It is well exhibited among the Lower Silurian lavas of Snowdon, the Upper Silurian lavas of Dingle, and the Lower Old Red Sandstone lavas near Killarney.

Fig. 6.—Micropegmatitic or Granophyric structure in Granophyre, Mull (magnified).

Fig. 7.—Ophitic structure in Dolerite, Gortacloghan, Co. Derry (magnified).

A marked structure among some intrusive rocks, especially of an acid composition, is that calledMicropegmatiticorGranophyric. It consists in a minute intergrowth of two component minerals, especially quartz and felspar, and is more especially characteristic of certain granitic or granitoid rocks which have consolidated at some distance from the surface and occur as bosses, sills and dykes. It is also met with, however, in some basic sills. Examples of all these and other structures will occur in the course of the following description of British volcanic rocks.

(2) The second type of devitrification, conspicuous in rocks of more basic composition, is marked by a more complete development of crystallization. Among basic, as among acid rocks, there are proofs of the consolidation of definite minerals at more than one period. Where the molten material has suddenly cooled into a black glass, porphyritic felspars or other minerals are often to be seen which were already floating in themagma in its molten condition. During devitrification, however, other felspars of a later period of generation made their appearance, but they are generally distinguishable from their predecessors. Probably most basic and intermediate rocks, when poured out at the surface as lavas, were no longer mere vitreous material, but had already advanced to various stages of progress towards a stony condition. These stages are still to some extent traceable by the aid of the microscope.

Microlites of the component minerals are first developed, which, if the process of aggregation is not arrested, build up more or less perfect crystals or crystalline grains of the minerals. Eventually the glass may be so completely devitrified by the development of its constituent minerals as to be wholly used up, the rock then becoming entirely crystalline, or to survive only in scanty interstitial spaces. In the family of the basalts and dolerites the gradual transition from a true glass into a holocrystalline compound may be followed with admirable clearness. The component minerals have sometimes crystallized in their own distinct crystallographic forms (idiomorphic); in other cases, though thoroughly crystalline, they have assumed externally different irregular shapes, fitting into each other without their Proper geometric boundaries (allotriomorphic).

A specially characteristic feature of many basic rocks is the presence of what is termed anOphiticstructure (Fig. 7). Thus the component crystals of pyroxene occur as large plates separated and penetrated by small needles and crystals of felspar. The portions of pyroxene, divided by the enclosed felspar, are seen under the microscope to be in optical continuity, and to have crystallized round the already formed felspar. This structure is never found in metamorphic crystalline rocks. It has been reproduced artificially from fusion by Messrs. Fouqué and Michel Lévy.

The nameVarioliticis applied to another structure of basic rocks (Fig. 8), in which, especially towards the margin of eruptive masses, abundant spheroidal aggregates have been developed from the size of a millet-seed to that of a walnut, imbedded in a fine-grained or compact greenish matrix into which the kernels seem to shade off. These kernels consist of silicates arranged either radially or in concentric zones.

3.Flow-structureis an arrangement of the crystals, vesicles, spherulites, or devitrification-streaks in bands or lines, which sweep round any enclosed object, such as a porphyritic crystal or detached spherulite, and represent the curving flow of a mobile or viscous mass. Admirable examples of this structure may often be observed in old lavas, as well as in dykes and sills, the streaky lines of flow being marked as distinctly as the lines of foam that curve round the boulders projecting from the surface of a mountain-brook.

Flow-structure is most perfectly developed among the obsidians, rhyolites, felsites and other acid rocks, of which it may be said to be a frequently conspicuous character (Fig. 9). In these rocks it is revealed by the parallel arrangement of the minute hair-like bodies and crystals, or by alternate layers of glassy and lithoid material. The streaky lines thus developed are sometimes almost as thin and parallel as the leaves of a book. But theygenerally show interruptions and curvatures, and may be seen to bend round larger enclosed crystals, or to gather into eddy-like curves, in such a manner as vividly to portray the flow of a viscous substance. These lines represent on a minute scale the same flow-structure which may be traced in large sheets among the lavas. The porphyritic crystals and the spherulites are also drawn out in rows in the same general direction. Sometimes, indeed, the spherulites have been so symmetrically grouped in parallel rows that they appear as rod-like aggregates which extend along the margin of a dyke.

Fig. 8.—Variolitic or Orbicular structure, Napoleonite, Corsica (nat. size).

Among lavas of more basic composition flow-structure is not so often well displayed. It most frequently shows itself by the orientation of porphyritic felspars or of lines of steam-vesicles. Occasionally, however, sheets of basalt may be found in which a distinct streakiness has been developed owing to variations in the differentiation of the original molten magma. A remarkable and widespread occurrence of such a structure is met with among the Tertiary basalt-plateaux of the Inner Hebrides and the Faroe Islands. In the lower parts of these thick accumulations of successive lava-sheets, a banded character is so marked as to give the rocks the aspect of truly stratified deposits. The observer, indeed, can hardly undeceive himself as to their real nature until he examines them closely. As a full description of this structure will be given in a later chapter, it may suffice to state here that the banding arises from two causes. In some cellular lavas, the vesicles are arranged in layers which lie parallel with the upper and under surfaces of the sheets. These layers either project as ribs or recede into depressions along the outcrop, and thus impart a distinctly stratified aspect to the rock. More frequently, however, the banded structure is produced by the alternation of different varieties of texture, and even of composition, in the same sheet of basalt. Lenticular seams of olivine-basalt may be found intercalated in a more largely crystalline dolerite. These differences appear to point to considerable variations inthe constitution of the magma from which the lavas issued—variations which already existed before the discharge of these lavas, and which showed themselves in the successive outflow of basaltic and doleritic material during the eruption of what was really, as regards its appearance at the surface, one continuous stream of molten rock. It is impossible to account for such variations in the same sheet of lava by any process of differentiation in the melted material during its outflow and cooling. Analogous variations occur among the basic sills and bosses of the Tertiary volcanic series of Britain. These, as will be more fully discussed in later chapters, indicate a considerable amount of heterogeneity in the deep-seated magma from which the intrusive sheets and bosses were supplied (see vol. ii. pp.329,342).

Fig. 9.—Flow-structure in Rhyolite, Antrim, slightly reduced.

It is a common error to assume that flow-structure is a distinctive character of lavas that have flowed out at the surface. In reality some of the most perfect examples of the structure occur in dykes and sills, both among acid and basic rocks. Innumerable instances might be quoted from the British Isles in support of this statement.

Although, in the vast majority of cases, the presence of flow-structure may be confidently assumed to indicate a former molten condition of the rock in which it occurs, it is not an absolutely reliable test for an igneous rock. Experiment has shown that under enormous pressure even solid metals may be made to flow into cavities prepared for their reception.Under the vast compression to which the earth's crust is subjected during terrestrial contraction, the most obdurate rocks are crushed into fragments varying from large blocks to the finest powder. This comminuted material is driven along in the direction of thrust, and when it comes to rest presents a streakiness, with curving lines of flow round the larger fragments, closely simulating the structure of many rhyolites and obsidians. It is only by attention to the local surroundings that such deceptive resemblances can be assigned to their true cause.

Fig. 10.—Lumpy, irregular trachytic Lava-streams (Carboniferous), East Linton, Haddingtonshire.

4. TheDISPOSITION OF LAVAS IN SHEETS OR BEDSis the result of successive outflows of molten rock. Such sheets may range from only a yard or two to several hundred feet in thickness. As a rule, though with many exceptions, the basic lavas, such as the basalts, appear in thinner beds than the acid forms. This difference is well brought out if we compare, for instance, the massive rhyolites or felsites of North Wales with the thin sheets of basalt in Antrim and the Inner Hebrides. The regularity of the bedded character is likewise more definite among the basic than among the acid rocks, and this contrast also is strikingly illustrated by the two series of rocks just referred to. The rhyolites and felsites, sometimes also the trachytes and andesites, assume lumpy, irregular forms, and some little care may be required to trace their upper and under surfaces, and to ascertain that they really do form continuous sheets, though varying much in thickness from place to place (Fig. 10). Like modern acid lavas, they seem to have flowed out in a pasty condition, and to have been heaped up round the vents in the form of domes, or with an irregular hummocky or mounded surface. The basalts, and dolerites, and sometimes the andesites, have issued in a more fluid condition, and have spread out in sheets of more uniform thickness, as may be instructively seen in the sea-cliffs of Antrim, Mull, Skye, and the Faroe Islands, where the horizontal or gently-inclined flows of basalt lie upon each other in even parallel beds traceable for considerable distances along the face of the precipices (Figs.11,265, and286). The andesites of the Old Red Sandstone (Figs.99,100) and Carboniferous series (Figs.107,108,111,112,113,123) in Scotland likewise form terraced hills.

The length of a lava-stream may vary within wide limits. Sometimes an outflow of lava has not reached the base of the cone from the side of which it issued, like the obsidian stream on the flanks of the little cone of the island of Volcano. In other cases, the molten rock has flowed for forty or fifty miles, like the copious Icelandic lava-floods of 1783. In the basalt-plateaux of the Inner Hebrides a single sheet may sometimes be traced for several miles.

Fig. 11.—View at the entrance of the Svinofjord, Faroe Islands, illustrating the terraced forms assumed by basic lavas. The island on the left is Borö, that in the centre Viderö, and that on the right Svinö.

Some lavas, more especially among the basic series, assume in cooling aColumnar structure, of which two types may be noticed. In one of these the columns pass with regularity and parallelism from the top to the bottom of a bed (Figs.171,225). The basalt in which Fingal's Cave, in the isle of Staffa, has been hollowed out may be taken as a characteristic example (Fig. 266a). Not infrequently the columns are curved, as at the well-known Clam-shell Cave of Staffa. In the other type, the columns or prisms are not persistent, but die out into each other and have a wavy, irregular shape, somewhat like prisms of starch. These two types may occur in successive sheets of basalt, or may even pass into each other. At Staffa the regularly columnar bed is immediately overlain with one of the starch-like character. The columnar structure in either case is a contraction phenomenon, produced during the cooling and shrinking of the lava. But it is difficult to say what special conditions in the lava were required for its production, or why it should sometimes have assumed the regular, at others the irregular form. It may be found not only in superficial lavas but in equal perfection in some dykes and intrusive sills or injections, as among the Tertiary volcanic rocks of the island of Canna (Figs.307and308).

The precipitation of a lava-stream into a lake or the sea may cause the outer crust of the rock to break up with violence, so that the still molten material inside may rush into the water. Some basic lavas on flowing into water or into a watery silt have assumed a remarkable spheroidal sack-likeor pillow-like structure, the spheroids being sometimes pressed into shapes like piles of sacks. A good instance of this structure occurs in a basalt at Acicastello in Sicily.[1]A similar appearance will be described in a later chapter as peculiarly characteristic of certain Lower Silurian lavas associated with radiolarian cherts in Britain and in other countries (Fig. 12).

[1]See Prof. G. Platania in Dr. Johnston-Lavis'South Italian Volcanoes, Naples (1891), p. 41 and plate xii.

Fig. 12.—Sack-like or pillow-form structure of basic lavas (Lower Silurian), Bennan Head, Ballantrae, Ayrshire.

It probably seldom happens that a solitary sheet of lava occurs among non-volcanic sedimentary strata, with no other indication around it of former volcanic activity. Such an isolated record does not seem to have been met with in the remarkably ample volcanic register of the British Isles. The outpouring of molten rock has generally been accompanied with the ejection of fragmentary materials. Hence among the memorials of volcanic eruptions, while intercalated lavas are generally associated with sheets of tuff, bands of tuff may not infrequently be encountered in a sedimentary series without any lava. Instances in illustration of these statements may be culled from the British Palæozoic formations back even into the Cambrian system.

A characteristic feature of some interest in connection with the flow of lava is the effect produced by it on the underlying rocks. If these are not firmly compacted they may be ploughed up or even dislocated. Thus the tuffs of the Velay have sometimes been plicated, inverted, and fractured bythe movement of a flowing current of basalt.[2]The great heat of the lava has frequently induced considerable alteration upon the underlying rocks. Induration is the most common result, often accompanied with a reddening of the altered substance. Occasionally a beautifully prismatic structure has been developed in the soft material immediately beneath a basalt, as in ferruginous clay near the village of Esplot in the Velay, in which the close-set columns are 50 centimetres long and 4 to 5 centimetres in diameter.[3]Changes of this nature, however, are more frequent among sills than among superficial lavas. Many examples of them may be gathered from the Scottish Carboniferous districts.

[2]M. Boule,Bull. Cart. Géol. France, No. 28, tom. iv. (1892), p. 235.[3]M. Boule.Op. cit.p. 234.

[2]M. Boule,Bull. Cart. Géol. France, No. 28, tom. iv. (1892), p. 235.

[3]M. Boule.Op. cit.p. 234.

Variations of structure in single lava-sheets.—From what has been said above in regard to certain kinds of flow-structure among basic rocks, it will be evident that some considerable range of chemical, but more particularly of mineralogical, composition may be sometimes observed even within the same sheet of lava. Such differences, it is true, are more frequent among intrusive rocks, especially thick sills and large bosses. But they have been met with in so many instances among superficial lavas as to show that they are the results of some general law, which probably has a wide application among the surface-products of volcanic action. Scrope expressed the opinion that in the focus of a volcano there may be a kind of filtration of the constituents of a molten mass, the heavier minerals sinking through the lighter, so that the upper portions of the mass will become more felspathic and the lower parts more augitic and ferruginous.[4]

[4]Volcanoes, p. 125.

Leopold von Buch found that in some of the highly glassy lavas of the Canary Islands the felspar increases towards the bottom of the mass, becoming so abundant as almost to exclude the matrix, and giving rise to a compound that might be mistaken for a primitive rock.[5]

[5]Description Physique des Isles Canaries(1836), p. 190.

Darwin observed that in a grey basalt filling up the hollow of an old crater in James Island, one of the Galapagos group, the felspar crystals became much more abundant in the lower scoriaceous part, and he discussed the question of the descent of crystals by virtue of their specific gravity through a still molten lava.[6]

[6]Geological Observations on Volcanic Islands(1844), p. 117.

Mr. Clarence King during a visit to Hawaii found that in every case where he broke newly-congealed streamlets of lava, "the bottom of the flow was thickly crowded with triclinic felspars and augites, while the whole upper part of the stream was of nearly pure isotropic and acid glass."[7]This subject will be again referred to when we come to discuss the characters of intrusive sills and bosses, for it is among them that the most marked petrographical variations may be observed. Examples will be cited both from the intrusive and extrusive volcanic groups of Britain.

[7]U.S. Geol. Exploration of the Fortieth Parallel, vol. i. (1878), p. 716.

Volcanic cycles.—Closely related to the problem of the range of structureand composition in a single mass of lava is another problem presented by the remarkable sequence of different types of lava which are erupted within a given district during a single volcanic period. Nearly thirty years ago Baron von Richthofen drew attention to the sequence of volcanic materials erupted within the same geographical area. He showed, more especially from observations in Western America, that a definite order of appearance in the successive species of lava could be established, the earliest eruptions consisting of materials of an intermediate or average composition, and those of subsequent outflows becoming on the whole progressively more acid, but finishing by an abrupt transition to a basic type. His sequence was as follows: 1. Propylite; 2. Andesite; 3. Trachyte; 4. Rhyolite; 5. Basalt.[8]This generalisation has been found to hold good over wide regions of the Old World as well as the New. It is not, however, of universal application.[9]Examples are not uncommon of an actual alternation of acid and basic lavas from the same, or at least from adjacent vents. Such an alternation occurs among the Tertiary eruptions of Central France and among those of Old Red Sandstone age in Scotland.

[8]Trans. Acad. California, 1868. Prof. Iddings'Journ. Geol., vol. i. (1893), p. 606.[9]See Prof. Brögger, "Die Eruptivgesteine des Kristianiagebietes," part ii. (1895), p. 175;Zeitsch. Kryst. und Mineral, vol. xvi. (1890) p. 83. This author would, from this point of view, draw a distinction between rocks which have consolidated deep within the earth and those which have flowed out at the surface, since he thinks that we are not justified in applying our experience of the order of sequence in the one series to the other. Yet there can be no doubt that in many old volcanic districts the masses that may be presumed to have consolidated at a great depth have been in unbroken connection with masses that reached the surface. These latter, as Prof. Iddings has urged, furnish a much larger body of evidence than the intrusive sheets and bosses.

[8]Trans. Acad. California, 1868. Prof. Iddings'Journ. Geol., vol. i. (1893), p. 606.

[9]See Prof. Brögger, "Die Eruptivgesteine des Kristianiagebietes," part ii. (1895), p. 175;Zeitsch. Kryst. und Mineral, vol. xvi. (1890) p. 83. This author would, from this point of view, draw a distinction between rocks which have consolidated deep within the earth and those which have flowed out at the surface, since he thinks that we are not justified in applying our experience of the order of sequence in the one series to the other. Yet there can be no doubt that in many old volcanic districts the masses that may be presumed to have consolidated at a great depth have been in unbroken connection with masses that reached the surface. These latter, as Prof. Iddings has urged, furnish a much larger body of evidence than the intrusive sheets and bosses.

The range of variation in the nature of the eruptive rocks during the whole of a volcanic period in any district may be termed "a volcanic cycle." In Britain, where the records of many volcanic periods have been preserved, a number of such cycles may be studied. In this way the evolution of the subterranean magma during one geological age may be compared with that of another. It will be one of the objects of the following chapters to trace out this evolution in each period where the requisite materials for the purpose are available. We shall find that back to Archæan time a number of distinct cycles may be observed, differing in many respects from each other, but agreeing in the general order of development of the successive eruptions. Leaving these British examples for future consideration, it may be useful to cite here a few from the large series now collected from the European continent and North America.[10]

[10]Prof. M. Bertrand in a suggestive paper published in 1888 dealt with the general order of appearance of eruptive rocks in different provinces of Europe. But the materials then at his command probably did not warrant him in offering more than a sketch of the subject,Bull. Soc. Geol., France, xvi. p. 573. In the same volume there is a paper by M. Le Verrier, who announces his opinion that the eruption of the basic rocks takes place in times of terrestrial calm, while that of the acid rocks occurs in periods of great disturbance,op. cit.p. 498. Compare also Prof. Brögger,Die Eruptivgesteine des Kristianiagebietes, ii. p. 169.

Among the older rocks of the European continent, Prof. Brögger has shown that in the Christiania district the eruptive rocks which traverse the Cambrian and Silurian formations began with the outburst of basic materialsuch as melaphyre, augite-porphyrite, and gabbro-diabase, having from about 44 to about 52 per cent of silica. These were followed by rocks with a silica-percentage ranging from about 50 to 61, including some characteristic Norwegian rocks, like the rhomben-porphyry. The acidity continued to increase, for in the next series of eruptions the silica-percentage rose to between 60 and 67, the characteristic rock being a quartz-syenite. Then came deep-seated protrusions of highly acid rocks, varieties of granite, containing from 68 to 75 per cent of silica. The youngest eruptive masses in the district show a complete change of character. They are basic dykes (proterobase, diabase, etc.).[11]

[11]Eruptivgest. Kristianiageb., 1895.

The same author institutes a comparison between the post-Silurian eruptive series of Christiania and that of the Triassic system in the Tyrol, and believes that the two cycles closely agree.[12]

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