I SLA. GEOLOGY. 



and by the position of these, the laminar structure is deter- 

 mined. Whatever quartz may be present, is mixed with 

 the felspar. It is worthy of remark, that where gneiss 

 occurs with argillaceous schist, it generally presents ana- 

 logous characters ; the felspar and quartz being inter- 

 laminated at the surfaces of change with the argillaceous 

 ingredient, and towards the more distant parts, with horn- 

 blende. Without attention, the two are easily confounded. 



This fact is interesting when compared with the ana- 

 logous transitions, already noticed, which occur between 

 argillaceous and hornblende schist, and which also 

 appear to prevail in the vicinity of gneiss. An analogy 

 equally interesting is presented by a circumstance visible in 

 the neighbourhood of Cruachan. In this place, where the 

 argillaceous schist is traversed by granite veins, it is found 

 to undergo a change of character at the junction, passing 

 into hornblende schist for a limited distance. The frag- 

 ments which are imbedded in the granite, and which, from 

 their forms and position, appear unquestionably to be 

 detached portions of the same strata, also consist of 

 hornblende ; while in certain cases it is even possible 

 to trace the gradual progress from the argillaceous and 

 earthy matter, to the crystallized form and change of 

 substance exhibited in the hornblende. The inferences 

 that may be deduced in all these cases are obvious ; and 

 they are strongly confirmed by the analogous circum- 

 stances occurring in trap rocks, which are pointed out 

 on different occasions in this work. In these cases the 

 schists are also converted into basalt, or into other varieties 

 of trap of which hornblende is a predominant ingredient ; 

 apparently from the influence of those causes which it 

 is unnecessary again to suggest. 



The most interesting circumstances relating to the clay 

 slate and quartz rock of Isla having been thus largely 

 dwelt on, in considering the western side of the island, 

 little remains to be said of the same rocks as they are 

 found occupying the eastern ridge. Alternations of a 



