261. Proofs of the igneous formation of whinstone, still more direct, are derived from the induration of the contiguous strata; from their disturbance when interfered by veins of whinstone; and from the charring of the coal which happens to be in contact with these veins. These are considered above at § 66, 67, &c.; and it is particularly taken notice of at § 66, that pieces of sandstone are sometimes found as if floating in the whinstone, and, at the same time, greatly altered in their texture. One of the best and most unequivocal instances of this sort which I have seen, is to be found on the south side of _Arthur's Seat_, near Edinburgh. The rock which composes the upper part of the hill, on that side, is a whinstone breccia, such as we have many examples of, and, I believe, very much resembling what is called a _lava brecciata_ by the volcanic geologists. The stony fragments included in this compound mass, are for the greater part rounded; and some of them are of whinstone, others of porphyry, strongly characterized by rectangular maculæ of feldspar, and many seem to be of sandstone, but so considerably altered, as to leave it at least disputable whether they really are so or not. In one part, however, where the face of the rock is nearly perpendicular, a narrow ridge is seen standing out from the rest, and of a different colour, being more entirely covered with moss than the rock round about it, and, as may be presumed from that circumstance, less liable to decomposition. On examination I found, that this ridge does not consist of whinstone, but of a very hard and highly consolidated sandstone. It appears to be the edge of a stratum, of the thickness of about nine or ten inches, and of the height of fifteen or sixteen feet. It is not perfectly straight, but slightly waved, its general direction being nearly vertical; and it is on both sides firmly embraced by the whinstone. When broken, it appears that this sandstone resembles in colour, and in every thing but its greater consolidation, and more vitreous structure, the common grit found at the bottom of the hill, and over all the adjacent plain.