2,54 JLA. GEOLOGY. 



scarcely deserving" the name of hills, lying on the outer 

 parts of the slate, and in a direction corresponding to that 

 of its beds. It is very irregular in aspect and form, being 

 split into fragments similar to those of the schist which it 

 accompanies ; and it might for that reason be readily 

 overlooked. The basis of this rock is a blueish indurated 

 clay, very much like the schist with which it is associated, 

 and the crystals of felspar, of a pink colour, are very 

 crowded, and generally about the size of a small pea. 

 The resemblance of this substance however to the por- 

 phyries is quite superficial, since the masses, notwith- 

 standing their appearance of independence, are portions 

 of beds appertaining to the schistose rocks. 



Having thus terminated the account of the principal 

 rocks which enter into the composition of Isla, it only 

 remains to describe the veins, and the independent 

 minerals. 



The most numerous and conspicuous veins are those of 

 trap, which are found throughout the island, but are parti- 

 cularly abundant on the shore between Balachar and 

 Macarthur's head, and between Ardtala and Ardmore. 

 Their courses are continuous for a great space, and may 

 often be traced almost across the whole country. The 

 remarks however already made in describing those of Jura, 

 supersede the necessity of dwelling on them here : a few 

 particulars will suffice. Their direction is various, and 

 their breadth varies from forty or fifty feet to ten. In 

 general they are more permanent than the surrounding 

 rocks ; in consequence of which they are traced by their 

 protrusion, and not, as in many other places, by the cavi- 

 ties which they have left. The permanence of these veins 

 has here in many cases, as well as in Jura and in Mull, 

 produced very remarkable appearances ; the high walls of 

 4;rap remaining in the form of ruined castles or buildings. 

 The quality of the veins varies, but the most prevalent 

 is a black basalt. This is often amygdaloida^ containing 

 zeolites ; in a few instances it passes into greenstone, and 



