Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/282

 and to ascertain the real nature of this kind of alternation, since it has been quoted at different times as proving a regular alternation between granite and schist, the limestone, as it happens, having been here overlooked. It is no further an alternation than because these substances, which in numerous other instances are confounded and irregularly mixed together, happen in this particular one and for a very short space, to have assumed a disposition accidentally more regular. The question of alteration must rest on other facts than these. A blank of about eighty yards follows this rock, and is succeeded by one of the most remarkable junctions of the different rocks already described, which occurs in the course of the river. The main body of this compound mass consists of white limestone or marble, without any tendency to that regularly bedded form which is the general characteristic of the limestone in Glen Tilt. It is accompanied by a small portion of schist as well as by a mass-of granite, with both of which it is variously intermixed. The marble itself is in most places of a pure white, a fine grain, and dry aspect, and is extremely hard. In some places it is of an ochry colour, and is interspersed by thin veins of the same substance, so as to resemble some of the palest and worst specimens of that marble known by the name of Giallo Antico. The schist which accompanies it is argillaceous and of a bluish colour, and the granite, both in the larger masses and in the ramifications, bears so near a resemblance to that formerly described that it is superfluous to describe it again. The veins which traverse the marble, like those at the bridge above mentioned, are of various sizes and are placed in every possible direction, but the ramifications are neither so numerous nor so minute as in that instance, nor is there any appearance of the laminated