470 OVERLYING ROCKS. 



A more particular account of such veins is given 

 in the seventh chapter. The other interesting- 

 circumstances which attend these veins, must be 

 referred to a geological history of these rocks ; 

 and it is pnly further necessary to add, that the 

 Western Islands of Scotland present, in a very dis- 

 tinct and accessible form, every circumstance re- 

 quisite for the illustration of that history. 



The rocks of this family present many remark- 

 able varieties of structure, of which the general 

 details have already been given in the chapter on 

 that subject. A few more particulars are here 

 necessary, as they form a very important part of 

 the history of these rocks, and are, in a few 

 instances, such, as, when combined with the 

 mineral characters, to lead to dangerous errors in 

 geological investigation. 



They often resemble granite in this respect, 

 being disposed in large beds, either straight or 

 curved, and divisible into prismatic and cuboidal 

 masses. When these have been rounded by the 

 weather, the rocks are not distinguishable by the 

 eye from granite ; and the deception is often 

 much increased by similarity of mineral charac- 



