Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/479

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469
Dr. Mac Culloch on Quartz Rock.

The ridge of Ben Lawers, it is true, (that ridge which does actually bound the valley of the Tay) consists of chlorite slate, with mica slate, and the other rocks of this character which need not be described. But the valley of the Lyon exhibits a formation totally different. The river Lyon runs with a very gentle fall through the greater part of its course into the Tay, proving that there is no very great difference of level between their respective vallies. In passing through Glen Lyon, the mica slate may be seen terminating on the south side of the river, in some places indeed long before it meets the bottom of the southern side of the valley. It is also obvious that the boundary of almost the whole northern side, as far at least as from Meggarney to Fortingall, consists of a sort of compact sandstone or “granular quartz,” a rock in no way differing from many of the varieties of quartz rock already described. This is the ridge which declines from the south of Schihallien, and which is intimately connected with the main ridge of that mountain. The sandstone itself exhibits a texture of infinite variety, but in all cases it possesses the same aspect of antiquity and induration which characterizes the rocks of Jura and Schihallien. It would be necessary to traverse the whole of this ridge to the top of Schihallien before we could ascertain positively whether the rock which forms its summit is connected with that of Glen Lyon, and what is the mode of this connexion; a task of considerable difficulty. Yet I have very little doubt that they are parts of one great deposit, similar to that which constitutes the north of Scotland and the Island of Jura.

Future and more extended investigations may perhaps enable us to assign even from this spot the relative antiquity and position of a rock hitherto but ill understood, and possibly from the more accessible parts of Schihallien determine the very important fact of its inferiority to, or alternation with, mica slate, a fact which however