Dr. Mac Ccjlloch oji Qtiartz Rock, 469 



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 des- 

 cribed. But the valley of theLyon 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 differ- 

 ence 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 accessi- 

 ble parts of Schihallien determine the very important fact of its 

 inferiority to, or alternation with, mica slate, a fact which however 



