Dr. Mac Culloch's Supplementary Observations^ &c. 265 



masses are sometimes truly porphyritic, containing crystals of felspar, 

 while at other times they consist of an uniform reddish compact 

 felspar, the usual basis of those porphyries which are found associated 

 with the primary rocks. There is much difficulty in determining 

 the true nature of these porphyritic masses. Occasionally they seem 

 to cut the strata, while in other places there can be no question that 

 they are parallel to them. Nevertheless I am still inclined to think 

 that even those which are occasionally seen conforming to the posi- 

 tion of the strata in which they lie, are, like the masses of trap 

 which have a direction parallel to the associated beds, only veins, of 

 which the direction is so far parallel to the position of the beds in 

 which they are found. 



From Loch Rannoch the quartz rock is seen crossing the moun- 

 tains to Loch Ericht, accompanied in the same way with the 

 various schists, and finally terminating in different places round the 

 outskirts of Ben Vualach, where its junction with the granite of 

 this mountain is visible. It may then be traced along the southern 

 border of Loch Ericht, where it joins with a similar series of rocks 

 that will be found extending from Dalwhinnie along the course of 

 the Truim, and then along that of the Garry all the way to Blair. 

 A fact of some importance is visible at Loch Ericht which 1 will 

 mention here, although not particularly connected with the history 

 of quartz rock. About half way between the top and bottom of the 

 lake on the south side, a large slide of the mountain is to be seen ; 

 the ruin is still so entire, of such magnitude, and so little encumbered 

 with recent accumulations of soil, that there is no difficulty in tracing 

 the fallen masses to the broken summits whence they were detached. 

 The quartz rock here, as in all the outskirts of this granitic country, 

 is traversed by granite veins. A few fallen stones have formed a 

 sort of cave capable of containing three or four persons, and known 



