Chapter 38

[355]The central portion of the Ochils was mapped for the Geological Survey by Mr. B. N. Peach, Prof. James Geikie, Prof. J. Young, Mr. R. L. Jack and myself.

No necks of agglomerate have been observed in this part of the chain. It will be seen from the section that the lowest visible parts of the Ochil volcanic series are here truncated by a fault which brings in the lower part of the Carboniferous system. By a curious conjuncture, immediately on the south side of this fault, a band of tuff appears, lying on the platform of the Carboniferous "plateau-lavas," to be hereafter considered, and passing below the well-known Hurlet seam of the Carboniferous Limestone, while through these strata rises one of the puys belonging to the second phase of volcanic activity in Carboniferous time in Scotland.

The best sections to show the nature and sequenceof the volcanic series of the Ochil Hills are to be observed at the west end of the chain. But as the whole succession of rocks cannot conveniently be obtained along one line, it is better to make several traverses, starting in each case from a known horizon. In this way, by means of three parallel sections, we may obtain the whole series of lavas and tuffs in continuous order. The first line of section starts in the lowest part of the tuffs represented at the bottom of the group inFig. 80, and runs up to the first thick ashy intercalation among the lavas. Following this bed south-westward to the Burn of Sorrow, we make from that horizon a second traverse across the strike to the summit of King's Seat Hill (2111 feet above the sea), where we meet with a well-marked lava which can be traced south-westwards, gradually descending the southern escarpment of the hills until it reaches the boundary fault near the village of Menstrie. Starting again from this definite horizon, we take a third line across the top of Dumyat (1373 feet) to the plain of Sheriffmuir, and there pass beyond the volcanic series into the overlying red sandstones. Arranged thus in continuous vertical sequence the succession is found to be as represented inFig. 81. The total thickness of volcanic material amounts to more than 6500 feet.

Fig. 81.—Diagram of the volcanic series of the Western Ochil Hills.The bands with vertical lines are various lavas (a); the tuffs and volcanic breccias are shown by the dotted bands (b); the uppermost portion of the section above the last thick group of lavas consists of conglomerates and sandstones (c) with a sheet of lava.

In this vast pile of volcanic ejections the lavas are almost entirely andesites of the usual characters. They include many slaggy and amygdaloidal varieties, some beautiful porphyries with large tabular felspars, likewise the resinous or glassy variety already referred to as occurring above Airthrey Castle. Their upper and under surfaces show the same structure as already described in those of the coast-sections in the Montrose tract. They include also more acid lavas, like the pale pink decomposing felsites of the Pentland Hills.

The tuffs and conglomerates occur on many platforms throughout the succession of lava-sheets. They form the lowest visible part of the whole volcanic series, but they are most abundant towards the top, and are best displayed at the western end of the hills. In Dumyat they form a conspicuousfeature. The whole of that hill consists of a constant alternation of lavas (chiefly slaggy andesites, but including also one felsitic flow) with bands of coarse and finer tuff and volcanic conglomerate. The greatest continuous mass of this fragmental material is 600 or 700 feet thick. From the extraordinary size of its included blocks it obviously must have been formed of ashes, stones and huge pieces of lava ejected from some vent in the near neighbourhood. Some of the individual blocks in this mass are as large as a Highland crofter's cottage.

The uppermost lavas of Dumyat dip under a still higher series of coarse volcanic conglomerates entirely made up of andesitic debris and reaching a thickness of about 1000 feet. This enormous accumulation was probably due partly to the abrasion of exposed cones and lava-ridges, and partly to volcanic discharges of fragmentary materials. Yet it is worthy of note that even amidst these evidences of the most vigorous volcanic activity we have also proofs of quiet sedimentation and traces of the fishes that lived in the waters of the lake. This particular zone of coarse conglomerate as it extends in a south-westerly direction becomes finer, and its upper part passes into a chocolate-coloured sandstone which has been quarried at Wolfe's Hole, Westerton, Bridge of Allan, at a distance of about three miles from where the line of section runs, which is embodied in the diagram,Fig. 81. It was from this locality that the specimens ofEucephalaspis,PteraspisandScaphaspiswere obtained which were described by Professor Ray Lankester.[356]

[356]Palæontographical Society, vols. xxi. (1867) and xxiii. (1869).

Above the last-named thick group of coarse volcanic conglomerates a solitary sheet of dark slaggy andesite may be observed. This lava is then overlain by the great depth of chocolate-coloured and red sandstones and marls of the plain of Strathmore (cinFig. 81). Nevertheless a few hundred feet up in these sedimentary deposits we meet with yet one further thin sheet of lava—the last known eruption of the long volcanic history of this district.

Before quitting the Ochil range I may refer to the evidence there obtainable as to the horizontal extent of separate sheets of lava. The western end of this range affords great facilities for following out individual beds of andesite along the bare terraced front of the great escarpment. Thus, the easily recognizable porphyrite which caps King's Seat Hill, above Tillicoultry (seeFig. 68), can be traced winding along the hill-slopes until it descends to the plain, and is then lost under the great fault, at the foot of Dumyat—a distance of more than six miles. There is, therefore, no difficulty in supposing that from the Ochil line of vents streams of lava should have rolled along the floor of the lake across to the base of the Highland slopes, 10 or 12 miles distant. We cannot tell, of course, whether any buried vents lie below the plain of Strathmore, but certainly no unquestionable trace of vents has yet been found among the crystalline rocks along the borders of the Highlands.[357]

[357]Allusion has already been made to the possible connection of the younger Highland granites with the volcanic series of the north-eastern part of Lake Caledonia; also to the occurrence of isolated masses of breccia piercing the crystalline schists near Loch Lomond (ante,p. 272).

Reference has already been made to the comparative scarcity of sills in this region, and to the occurrence of the acid group of Lintrathen porphyry and the more basic sheets between the Firth of Tay and Forfar. This scarcity no doubt arises in part from the extent to which the rocks that underlie the volcanic series are concealed. Yet it is noteworthy that along the coast-section of these rocks near Stonehaven hardly any intrusive sheets are to be seen.

It is unfortunate that the Ochil chain should be broken across and buried under younger formations at the very place where some of the most interesting vents in the whole area of the Old Red Sandstone might have been looked for.[358]We have to pass westwards across the Firth of Clyde to the Isle of Arran before we again meet with rocks of the same age and character.

[358]The Ochil area is not the only example of the abrupt termination of a volcanic band near its centre owing to faults or overlaps. The sudden disappearance of the Pentland lavas and tuffs on the northern side of the Braid Hills is another striking illustration.

In the course of the recent work of the Geological Survey in that island, Mr. W. Gunn has discovered that the Lower Old Red Sandstone includes some interstratified volcanic rocks on the north side of North Glen Sannox, and he has supplied me with the following notes regarding them. "The area in which the volcanic intercalations occur is much faulted and only a part of it has been mapped in detail, but the position of the interbedded igneous rocks is quite clear. The Old Red Sandstone here consists of three distinct members, the lowest of which is made up of coarse, well-rounded conglomerates, alternating with sandstones and purple mudstones. Above this, and apparently unconformable to it, is a middle series of light coloured conglomerates and sandstones, the pebbles in which are mainly of quartz. Finally comes an upper series of red sandstones and conglomerates, which occupy nearly the whole of the coast section, and it is this series which has generally been taken as the typical Old Red Sandstone of the island. The volcanic series is intercalated between the middle and upper divisions given above, and may be seen in several places on the hillside between the shepherd's house at North Sannox and Laggan. It consists mainly of old lava-beds of a dull reddish or purplish colour, often soft, and in places much decomposed. It seems basic in character. A specimen from near the Fallen Rocks, examined by Mr. Teall, was found to be too much altered for precise determination, but was probably a basalt originally. These rocks do not occur on the coast."

In the southern extremity of Cantyre some important relics of the volcanic rocks of the Lower Old Red Sandstone have been recently detected and mapped for the Geological Survey by Mr. R. G. Symes.[359]This division of the system has been ascertained by him to be extensively developed to the south of Campbeltown, and to include some small but interesting remains of the volcanic action which was so marked a feature in the areas of Lake Caledonia, lying further to the east. To the student of volcanic geology, indeed, this small tract at the extreme southern end of Argyllshire has a peculiar interest, for in no other part of the British Isles have the phenomena of the eruptive vents of the Lower Old Red Sandstone been more admirably laid bare. Not only are there necks in the interior like that represented inFig. 82; but others have been dissected by the waves along the southern shore, and their relations to the deposits of fragmentary material showered over the bottom of the lake have been more or less clearly exposed.

[359]The late Prof. James Nicol published in 1852 an account of the geology of the southern portion of Cantyre. He grouped all the igneous rocks of the district as one series, which he regarded as later than the Coal-formation and possibly of the same age as those of the north-east of Ireland. He made no distinction between the Lower Old Red Sandstone and the younger unconformable conglomerates (Quart. Journ. Geol. Soc.vol. viii. (1852), p. 406).

Fig. 82.—View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of Lower Old Red Sandstone age, about 400 yards wide in its longer diameter.

At Keil Point, a little to the east of the most southerly headland of the Mull of Cantyre, some reddish and purplish highly felspathic sandstones (ainFig. 83) dipping towards the east are found to pass upward into coarse volcanic breccias (b), which, followed eastwards, lose almost all trace of stratification, and are then abruptly succeeded by a neck of coarse agglomerate (c) measuring 25 yards from north to south, where its limits can be seen, and at least 12 yards from west to east. It is hardly possible to distinguish between the breccias to the west and the agglomerate of theneck, except by the rude bedding of the former which pass down into the well-bedded sandstones.

The agglomerate is a thoroughly volcanic rock. The materials consist chiefly of angular blocks of a pale purplish or lilac highly porphyritic mica-porphyrite, with large white felspars and hexagonal tables of black mica. These blocks might sometimes be mistaken for slags from their cavernous, weathered surfaces, but this rough aspect is found on examination to be due to the decay of their felspars.

Fig. 83.—Section of volcanic series on beach, Southend, Campbeltown.a, Fine reddish and purplish highly felspathic sandstones, largely composed of porphyry-debris and passing up into coarse breccias;b, volcanic breccias, coarse and only rudely stratified, formed of blocks of porphyry, sandstone fine tuff and andesite, together with water-worn quartzite pebbles derived from some conglomerate;c, coarse unstratified agglomerate forming a neck.

Perhaps the most singular feature among the contents of this neck is the number of well-rounded and smoothed pebbles and boulders of quartzite. These are dispersed at random through the mass, and are often placed on end. There can be no doubt that they are water-worn stones, but the contrast of their smooth surfaces and rounded forms with the rough angular blocks of igneous material is so striking as to lead at once to the conclusion that they cannot have acquired their water-worn character in the deposit where they now lie. Their positions and their occurrence with ejected volcanic blocks suggest that they too were discharged by volcanic explosions. They so exactly resemble the quartzite boulders and pebbles in the neighbouring Old Red Conglomerates that there can be little hesitation in regarding them as derived from these conglomerates. They seem to me to have come from a lower part of the Old Red Sandstone, which was shattered by volcanic energy either before the conglomerates were firmly consolidated or afterwards by such violent explosions as served to separate the pebbles from the matrix of the rock.

There occur also in the agglomerate blocks of fine tuff and ashy sandstone sometimes four feet long, and often stuck on end, showing that the deposits of earlier eruptions were broken up during the drilling of this little vent.

A few hundred yards further east a larger neck rises on the beach, immediately to the south of the old Celtic chapel of St. Columba. It consists also of exceedingly coarse agglomerate, with andesite blocks three and four yards in diameter. It is about 125 yards broad from east to west, on which sides it is seen to be flanked by coarse volcanic breccias and conglomerates, resembling in composition the materials of the neck, but showing an increasingly definite stratification as they are traced eastward in the ascending succession of deposits. Following the section in still the same easterly direction along the coast, we find that bands of fine felspathic sandstone, marking probably intervals of quiescence, are againand again succeeded by coarse brecciated conglomerates of igneous materials, which may be inferred to have been due to a renewal of violent eruptions. By degrees the evidence of stratification and of attrition among the volcanic materials becomes more pronounced as the ascending section is followed; blocks of andesite, even 18 inches or two feet in diameter, assume well-rolled, rounded, water-worn forms, like the pebbles of quartzite associated with them, and eventually the strata return to the usual aspect of the conglomerates of the district.

I have never seen anywhere better proofs of volcanic explosions, contemporaneous with a group of strata, and of the distribution of volcanic fragmentary material round the vents. A further point of much interest is the additional evidence furnished by this shore-section of considerable wave-action during the accumulation of the coarse conglomerates. To give to blocks of porphyrite two feet in diameter a smoothed and rounded form must have required the action of water in considerable agitation.

From the volcanic breccias and conglomerates of the Mull of Cantyre to the coast of Antrim in a straight line is a distance of little more than twenty miles. On a clear day the Old Red Sandstone of Cross Slieve, and the range of cliffs in which it abruptly descends to the sea between Cushendall and Cushendun, can be distinctly seen from the Argyllshire shore. The geologist who passes from the Scottish to the Irish sections cannot fail to be impressed with the resemblance of the rocks in the two countries, and with the persistence of the types of conglomerate in Lake Caledonia.

A picturesque section has been laid bare between the Coastguard Station south of Cushendall and Cushendun Bay.[360]At the south side of the little inlet of Cushendall, a compact dull quartz-porphyry is exposed in crags along the shore. This rock ranges in colour from dark brown and purple to pale-green and buff. Its texture also varies, as well as the proportion of its felspar-crystals and quartz-blebs. Some parts have a cavernous structure, like that of an amygdaloid, the small globular cavities being filled with green decomposition products.

[360]For descriptions of this district see J. Bryce,Proc. Geol. Soc.i. (1834) p. 396, v. (1837) p. 69; J. Kelly,Proc. Roy. Irish Acad.x. (1868), p. 239. The area is contained in Sheet 14 of the Geological Survey of Ireland, and was mapped by Mr. A. M'Henry and described by him in the accompanying Explanatory Memoir (1886), pp. 12, 25.

The stratigraphical relations of this rock are not quite clear, but it is certainly older than the Old Red conglomerates which lie to the north of it, for these are largely made up of its fragments. The matrix of these detrital masses consists mainly of the comminuted debris of the porphyry. The pebbles include all the varieties of that rock, and are tolerably well-rounded. There is no distinct evidence of volcanic action among these conglomerates. They resemble, however, many of the conglomerates in the Midland Valley of Scotland, which, as in the case of those on the Forfarshire and Kincardineshire coast, are in great part made of the detritusof andesitic lavas. The Cushendall rocks become coarser as they are traced northwards into lower members of the series, while at the same time the proportion of porphyry-debris in their constitution diminishes, and materials from the metamorphic series take its place. Thus at Cushendun the percentage of quartz-pebbles rises to 70 or 80. These blocks, of all sizes up to two feet or more in diameter, are admirably rounded and smoothed, like those in the Stonehaven section and those among the conglomerates at the south end of Cantyre. Fragments of the porphyry, however, still continue to appear, and the matrix shows an admixture of the finer detritus of that rock. I may remark in passing that no conglomerates of the Old Red Sandstone show more strikingly than these at Cushendun the effects of mechanical crushing subsequent to deposition and consolidation. In many parts of the rock it is hardly possible to find a rounded block that has not been fractured. Some of them, indeed, may be seen cut into half a dozen slices, which have been pushed over each other under the strain of strong lateral or vertical pressure.

In the interior of the country, after passing over the broad Tertiary basaltic plateau of Antrim, we come upon a large area of Lower Old Red Sandstone in Tyrone. It stretches from Pomeroy to Loch Erne, a distance of about 30 miles, and is about 12 miles broad. In lithological character the strata of this tract exactly resemble parts of the deposits of Lake Caledonia in Central Scotland. They include also a volcanic series which, down to the smallest points of detail, may be paralleled in the sister island.[361]This interesting westward prolongation of the volcanic record consists of a number of outlying patches confined to the eastern part of the district.

[361]This area of Old Red Sandstone is represented on Sheets 33, 34, 45 and 46 of the Geological Survey of Ireland, and the igneous rocks are described in the Memoirs on Sheets 33 (1886, p. 17) by Mr. J. R. Kilroe, and 34 (1878, p. 16) by Mr. J. Nolan.

The largest of these patches lies to the south of Pomeroy, where it forms a line of hills about four miles long, and covers an area of some five square miles. The rocks consist of successive sheets of andesite-lavas. These, as a rule, are not markedly cellular, though they include some characteristic amygdaloids. A distinguishing feature of some of the sheets is their remarkably well-developed flow-structure. Thus on Sentry Box, at the north-western end of the ridge, the fissility resulting from this structure so perfectly divides the rock into parallel flags that the material might easily be mistaken for a bedded rock. Where this structure has been produced in a cellular lava, the cavities have been drawn out and flattened in the direction of flow.

I have not observed true tuffs in any of the sections traversed by me in this district. But the conglomerates furnish abundant evidence of the contemporaneous outpouring of the lavas. Thus, in a brook a little west of Reclain, five miles south of Pomeroy, the section shown inFig. 84may be seen. At the base lies a coarse conglomerate (a) largely composed of andesite-debris, the stones being here, as elsewhere in the district, well rounded. Then comes a series of green and reddish highly-felspathic sandstones (b), followed by an exceedingly coarse conglomerate (c), formed mainly of the debris of andesites, especially lumps of slag. Some of the stonesmeasure 18 inches in diameter, and all are well water-worn. Immediately over this mass of detritus lies the lowest sheet of andesite-lava (d).

Fig. 84.—Section of the base of the volcanic series, Reclain, five miles south of Pomeroy.

Some sections visible in the neighbourhood of Omagh afford further evidence of volcanic action at the time of the deposition of the Old Red Sandstone of this region. At Farm Hill, a little to the east of the town, felspathic sandstones and breccias enclose angular and subangular pieces of various andesites, and occasionally even pieces of tuff. Near these strata a decayed andesite occurs in the bed of a stream, and a fresher variety is quarried at Farm Hill. A little further south another variety of andesite is exposed in two quarries at Recarson Meeting-House—a fine granular purplish-grey rock, with abundantly-diffused hæmatite pseudomorphs, probably after a pyroxene, and sometimes strongly amygdaloidal.

Fig. 85.Section of shales and breccias at Crossna Chapel, north-east of Boyle.aa, Green and grey shales;bb, green and grey hard sandstones and grits, some bands strongly felspathic;c, fine compact felspathic breccia, with angular chips of different felsites and andesites, etc.

There can thus be no doubt that this region of Ulster included several centres of volcanic activity during the deposition of the red sandstones and conglomerates, and that the lavas and volcanic conglomerates belonged to precisely the same types as those of the same geological age which occur so abundantly in Scotland.

Further south-west, near Boyle, in the county of Roscommon, certain curious felspathic breccias in the Old Red Sandstone have been mapped as "felstone."[362]So far as I have been able to examine them, however, they are entirely of fragmental origin. They contain pieces of andesitic and felsitic rocks, with fragments of devitrified glass, which undoubtedly point to the occurrence of volcanic eruptions during their deposition, though no tuffs and lavas appear to crop out in the narrow strip of the formation there exposed.

[362]See Sheet 66 Geological Survey of Ireland, and Explanation to that sheet (1878), p. 15. The rocks were previously described by Jukes and Foot,Journ. Roy. Geol. Soc. Ireland, vol. i. (1866), p. 249.

The accompanying section (Fig. 85) may be seen on the hills to the north-east of Boyle. Where quarried on the road-side to the north of Boyle, the series of deposits here represented contains a bed of coarse and exceedingly compact breccia, similar to that just referred to, but containing angular and subangular fragments six or eight inches long. The joints of these compact strata are remarkably sharp and clean cut, so that where the fragmentary character is not very distinct the rocks might easily be mistaken on casual inspection for felsites.

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