118 OVERLYING AND TRAP ROCKS. 



the materials is so rapid as to keep it in check, the 

 surface is formed of such fragments, to a great 

 deptli ; while, more generally, being productive of 

 fertile soils, the whole is obscured by the vegetable co- 

 vering. In the older porphyries, displaying projecting 

 rocks and precipices, it is often difficult to distinguish 

 between the mountains and those formed of the pri- 

 mary schists ; the characters being often also very 

 striking, as in Glenco. The more durable varieties 

 of trap sometimes also exhibit the peculiar spiry 

 forms of granite, as they resemble it in other respects ; 

 and I may refer for an example to the Cuchullin hills 

 of Sky. 



But they also present some peculiarities scarcely 

 found in any others ; among which is that of distinct 

 masses insulated on the surface of a district of strati- 

 fied rocks, or crowning, in single summits, some 

 mountain of another substance. These occur indif- 

 ferently on all other rocks, not excepting granite ; 

 while they consist, either of the older or of the more 

 recent members of this family. Ben Nevis presents 

 a remarkable example of an antient member of this 

 division, lying thus on granite ; and Morven displays 

 equally striking instances of a more recent one, 

 covering the modern strata which contain coal. In 

 other cases, the mountain outline consists of a suc- 

 cession of terraces, either horizontal or slightly in- 

 clined, bearing that resemblance to a gigantic stair- 

 case whence the term trap is supposed to have been 

 originally derived. This appearance arises from the 

 succcesive vertical fracture of the beds of which such 

 mountains are composed ; and where only one such 

 fracture occurs on the summit of a hill, another pe- 

 culiarity, by which these rocks are easily recognised 

 at a distance, is also produced ; in the long and rapid 



