on the Mountain Cruachan. 133 



of veins proceeding from the mass, rendered conspicuous by their 

 projection and superior durability, but in examining the surround- 

 ing rocks where in contact with them, no difference of structure or 

 composition is perceptible. I did not any where perceive a ten- 

 dency to the columnar form. 



Although the naked surfaces of these rocks might lead us to sup- 

 pose they were inimical to vegetation, this effect must rather be 

 attributed to the steepness of their declivities, which prevents the 

 accumulation of soil. From whatever cause it may arise, they seem 

 particularly subject to be destroyed by the action of the mountain 

 torrents, whence the enormous piles of fragments which annually 

 overwhelm the road and are fast raising the level of the valley. 



The simple rock, which, as I have already said, prevails over the 

 porphyritic varieties, is at one extreme a claystone, and at the other 

 a compact felspar, varying through several intermediate stages of 

 hardness. In the intermediate stages of transition to porphyry, a 

 single crystal only of felspar will sometimes be found in a large 

 fragment, the ultimate accumulation of which produces porphyries 

 of an infinite variety of aspects. Every variety of this substance 

 indeed, whether in colour or composition, which occurs in veins 

 throughout Scotland, is here found mixed together in the mass, 

 sometimes placed side by side with a sudden and decided transition, 

 at others graduating into each other by imperceptible degrees. The 

 colours graduate into each other in these cases just as do the dif- 

 ferent structures, and among these gradations the most striking are 

 those where black passes into red. It would be an useless task to de- 

 scribe the varieties of colour which occur, but the different shades of 

 grey, purple, and red, are the predominant ones. In some cases 

 dark blueish specimens are found veined with red, producing very 

 beautiful and remarkable varieties : in others, the red colour is so 



Vol. iv. s 



