194. This last is, I believe, the most northern junction which has been observed in our island. The western coast furnishes several more, which however are not all visible. The line of separation, between the primary schistus of the Grampians and the sandstone which covers it, is intersected at its western extremity by the Frith of Clyde, not far from Ardencaple in Dunbartonshire. The two kinds of stone can be traced within a few yards of each other, but not to the actual contact: the beds of sandstone nearest the schistus form as usual a breccia, loaded with fragments of the primary rock. The secondary rock, which begins here, continues for about fifty miles south, to Girvan in Ayrshire, where the primary schistus again rises up, but is not seen in contact with the secondary. It extends to the Mull of Galloway and the shores of the Solway Frith.
The Isle of Arran, however, not far distant from this part of the coast, contains a junction at its northern extremity, where secondary strata of limestone lie immediately on a primary micaceous schistus. This is described by Dr Hutton, and was the first phenomenon of the kind which he had an opportunity of examining.[97] The junction is visible but at one spot, and is not seen so distinctly as in some of the instances just mentioned; but the great quantity of pudding-stone near it, renders it more interesting than it would be otherwise. As the greater part of this little island is surrounded by secondary strata, other junctions might be expected to be visible.
[97] Theory of the Earth, vol. i. p. 429.
195. On the coast of England and Wales, from the Solway Frith to the Land's End, though there are several alternations from secondary to primary strata, I know not that any of them have been observed. At St Bride's Bay, in Pembrokeshire, the primary and secondary strata are seen very near their junction; but the precise line I believe is not visible. The coal-pits in the secondary strata, approach here within a few hundred yards of the primary. The secondary strata which commence at this place, occupy both sides of the Bristol Channel, and meet the Cornish schistus, which extends across the north of Devonshire to the Quantock Hills, in a line that may be looked for on the sea coast, some where between Watchett and Minehead.
196. Besides the sea coast, the beds of rivers may be expected to afford information on this subject. To the instances I have mentioned, I have accordingly two others from the inland country to be added. One of them is from the river Jed, a little way above Jedburgh, where the secondary strata are seen lying horizontally on the primary, a section of both being made by the bed of the river. The phenomena here are very distinct, and strongly marked; Dr Hutton has described and represented them in a plate.[98] He has mentioned another junction, not far from this, which he saw in the Tiviot. Both these belong to the same primary ridge with the Siccar point.
[98] Theory of the Earth, vol. i. p. 430; also plate 3.
197. I shall mention only one other, which was discovered by Lord Webb Seymour and myself, at the foot of the high mountain of Ingleborough, in Yorkshire. As we went along the Askrig road from Ingleton, about a mile and a half from the latter, an opening appeared in the side of the hill, on the right, about one hundred yards from the road, formed by a large stone, which lay horizontally, and was supported by two others, standing upright. On going up to the spot, we found it was the mouth of a small cave, the stone lying horizontally, being part of a limestone bed, and the two upright stones, vertical plates of a primary argillaceous schistus. The limestone bed, which formed the roof of the cave, was nearly horizontal, declining to the south-east; the schistus nearly vertical, stretching from north-west by west, to south-east by east. The schistus, though close in contact with the limestone, seemed to contain nothing calcareous, and did not effervesce with acids in the slightest degree.
As this cave is at the foot of Ingleborough, a cold wind, 24° below the temperature of the external air, which issued from the mouth of it, might very well be supposed to come from the inmost recesses of that mountain. Ingleborough, which consists entirely of strata of limestone and grit, nearly horizontal, and alternating with one another, rises to the height of 1800 or 2000 feet above the spot where we now stood. This, I believe, is the greatest thickness of secondary strata that has ever been observed incumbent on the primary, and it is therefore a geological fact highly deserving of attention. The country all round, to a very great extent, is composed of limestone, with a few beds of grit interposed, and forming, beside Ingleborough, some other high mountains, such as Wharnside and Pennigant, all resting, it is probable, on the same foundation.
At the spot just described, no breccia appeared to be interposed between the primitive and secondary rock; but we found a breccia at another point of the same junction, not far distant. This was at a cascade, in the river Greta, called Thornton Force, about two miles and a half from the place just mentioned. The Greta here precipitates itself from a horizontal rock of limestone; and, after a fall of about eighteen or twenty feet, is received into a bason which it has worked out in the primary schistus. This schistus is in beds almost perpendicular; it exactly resembles that which has Just been described, and stretches nearly in the same direction. On the south side of the river a breccia was seen, lying upon the schistus, or rather, it might be said, that the lowest beds of limestone contained in them many rounded fragments of stone, which, on comparison, resembled exactly the schistus underneath. The primary rock itself is here seven or eight hundred feet above the level of the sea.
The same schistus, somewhat lower down the valley, and nearer to Ingleton, appears in large quantities, and is quarried for slate. Here, however, the immediate junction of the limestone and schistus does not appear.