Dr. MacCullocho«//&^ Geology of Glen Tilt. 281 



the drawing as it is to me, and it is probably not very obvious to 

 any one. 



It is not far from this place, and on the left bank of the river, 

 that a greenstone vein may be seen traversing the strata at nearly 

 right angles. It is the only specimen of genuine greenstone which 

 I observed in Glen Tilt, and it appeared to me to resemble those 

 veins which are found in Cruachan and in the neighbouring moun- 

 tains, where they also frequently assume a porphyritic aspect. Still 

 however they differ considerably in their general appearance from 

 those greenstone veins which are most usual in Scotland, and which 

 are found to predominate in the vicinity of the larger formations 

 of trap in the Western islands, and in many well known parts of 

 the continent of Scotland. 



The great mass of limestone which we shall hereafter find 

 forming the whole of the left boundary of Glen Tilt, is of a dark 

 blue colour, with one or two exceptions which I have already 

 described in the progress down the river. But the beds at this 

 place are of various colours, and offer some of the most beautiful 

 ornamental marbles which Scotland has yet produced.* 



The basis of nearly the whole is a white, rather larger grained, 

 and crystalline marble. Beds of this variety occur in a pure state, 

 and of considerable dimensions. But as all these marbles contain 

 more or less of mica, with which substance they are interstratified, 

 the white colour is seldom pure, being mottled with the slight grey 

 tint which mica in similar cases always produces. It cannot there- 

 fore be considered as a statuary marble, since modern artists, ac- 



* Having pointed out the circumstance to the Duke of Atholl two years ago, quarries 

 are now opened in tliem, by which the numerous varieties which they contain have been 

 more completely brought to light tluui thfV could have been by the operation of a mine- 

 ralogist's hammer. 



Vol. III. 2n 



