OVERLYING AND TRAP ROCKS. 135 



taught, among the number who had determined that 

 there was nothing more to learn ; and that the science 

 is thus benefited, though it be only by the repetition 

 of identical evidence. If 5 at times, these rocks ex- 

 hibit no traces of a peculiar configuration, breaking 

 indifferently in any direction, and with faces of no 

 character, this is more particularly true of the syenites 

 and claystones ; though occurring, perhaps to an 

 equal extent, in the porphyries, of whatever date. In 

 other examples, they present an irregular and angular 

 fracture, so as to leave acute projecting edges or 

 points ; as seems more particularly true of augit rock, 

 and of the analogous greenstones. Very rarely, they 

 are disposed in curved or straight beds, subsequently 

 divided into irregular prismatic masses ; as I have 

 already observed of Hypersthene rock. This feature 

 is not however peculiar to the more antient traps, as 

 some have supposed ; since that of the Corstorphjrie 

 hills, which lies above the coal series, can scarcely be 

 distinguished from granite in this respect. The 

 large laminar structure is also not uncommon ; oc- 

 curring in the porphyries of all ages, as well as in the 

 syenites and claystones. It is sometimes vertical and 

 very distinct, at others, horizontal ; being then attri- 

 buted to stratification, as the similar forms in granite 

 have been, by those teachers of geology whom I am 

 too often compelled to notice. I need not repeat the 

 grounds of distinction ; and I have already shown 

 where there is a remarkable affinity with gneiss, in 

 the case of hypersthene rock. 



The vertical division into irregular and angular 

 parts, is one of the most striking configurations of 

 these rocks ; occurring in the most antient porphy- 

 ries and claystones, as in Glenco, but most conspi- 

 cuous in the traps that are associated with the secon- 

 dary strata, including basalt, augit rock, greenstone 



