122 Dr. Mac Culloch's observations 



granite nor with the schist, nor apparently producing any disturbance 

 in the course or direction of these rocks. They are, as I before re- 

 marked, of various colours and compositions, and two veins may often 

 be seen running parallel and in absolute contact with each other, with- 

 out interference or disturbance, the one of a dark red, and the other 

 of a light grey or some other colour, as represented in one of the 

 sketches.* They are so numerous that perhaps a fortieth or fiftieth 

 part of that region of Cruachan which I examined consists of por- 

 phyry veins. The principal varieties both of colour and composition 

 which I remarked, are the following, and their basis consists of that 

 rock which is now by general consent, called compact felspar, but 

 which has at times been designated by the term hornstone. 



Brick red porphyry, the base of compact felspar with imbedded 

 crystals of the same colour : a very few specks of white felspar 

 and of greenish hornblende are dispersed through it. 



A mixed granular basis of reddish-grey compact felspar, with 

 crystals of a larger size and paler colour, containing also grains of 

 pyrites and long slender crystals of hornblende in abundance. 



Base of an uniform dark grey compact felspar with crystals of 

 white felspar. 



The same, but with the addition of black mica, hornblende and 

 pyrites. 



A grey ground with very large crystals of pale grey felspar ; 

 these crystals themselves containing crystals of hornblende. The 

 base contains crystallized mica. 



A purple ground with crystals of brownish yellow felspar. 



A brown uniform ground with rare and minute crystals of 

 felspar. 



* Plate 6, fig. 2. 



