Fig. 80.—Generalized section across the heart of the Ochil Hills, from Dunning on the north to the Fife Coal-field near Saline on the south.
1. Volcanic tuffs and agglomerates; 2. Andesite lavas; 3. Lower Old Red Sandstone and conglomerate; 4. Necks of felsitic rocks; 5. Upper Old Red Sandstone and Calciferous Sandstones; 6. Representative of the Plateau lavas and tuffs of the Lower Carboniferous series; 7. Hurlet (Carboniferous) Limestone; 8. Dolerite sill; 9. Sandstones, shales and coals of the Carboniferous Limestone series; 10. Neck of the Puy series (Carboniferous); f, Fault.
A section across the centre of the Ochil chain,[355] from Dunning in Strathearn to the Crook of Devon and the Fife Coal-field, gives the structure which is generalized in [Fig. 80]. At the north end the volcanic series is found to be gradually split up into separate lava-sheets until it dips under the red sandstones of Strathearn. Traced southwards the rocks become entirely volcanic. Some of their most conspicuous and interesting members are pale felsitic tuffs, which occupy a considerable tract of ground about Craig Rossie, south-east of Auchterarder. As the dip gradually lessens the harder lavas are able to spread over wider tracts of ground, capping the hills and ridges, while underneath them thick masses of tuff and conglomerate are laid bare in the valleys. A number of bosses of orthophyre rise through these rocks and are accompanied by many veins and dykes of similar material. It is not improbable that some of these bosses, as already suggested, may represent vents. They are especially prominent among the hills due south of Auchterarder. One of these eminences, known as the Black Maller, is composed of a typical orthoclase-felsite without mica. Another, about four and a half miles further south, forms the conspicuous summit of Ben Shee overlooking Glen Devon, and consists of a similar rock with a characteristic platy structure.
[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 sequence of 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 in [Fig. 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 in [Fig. 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 conspicuous feature. 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 of Eucephalaspis, Pteraspis and Scaphaspis were obtained which were described by Professor Ray Lankester.[356]