OVERLYING AND TKAP RCfc'KS. 



slope resulting from the fall of the materials. These 

 latter features appear to occur peculiarly where these 

 rocks are incumbent on the secondary strata ; hut 

 they are not limited to one particular member of the 

 family, since they belong to basalt, to greenstone, and 

 to claystone. 



The rocks of this family are found incumbent on 

 every other rock from granite to chalk ; as the recent 

 members sometimes also cover the more ancient ones 

 in a similar manner. But the compound porphyries, 

 those with a base of compact felspar, and the simple 

 compact felspars, are, generally, in contact with the 

 primary rocks ; yet not exclusively, since the most 

 recent traps covering the secondary strata, sometimes 

 also occupy the same position, though these parti- 

 cular porphyries are seldom found upon the secon- 

 dary. Perhaps they never thus occur in extensive 

 masses and with decided characters throughout ; but, 

 as many of the most recent members approach in 

 character to those which appear more antient, it is 

 impossible to acquire absolute certainty in any case,, 

 without a critical examination of all the circumstances. 



If the forms of these great masses are very diver- 

 sified, so are their lateral extent and depth. They 

 sometimes cover a great range of country, as in 

 Spanish America ; while, even with us, the Cheviot, 

 the Campsie, the Pentland, and the Ochil hills present 

 considerable tracts. On the other hand, they fre- 

 quently occur in minute patches, or as insulated 

 mountain summits, the examples of which, in Scot- 

 land, are too common to demand quoting. Their 

 depths are equally various, that of the Cuchullin hills 

 reaching to 3000 feet at least ; while in Scotland they 

 range from that to a few feet in thickness ; though 

 in this case, as might be expected, their lateral extent 



