122 KERRERA. GEOLOGY. 



decomposition which they evidently possess. That faci- 

 lity is marked by the enormous waste they have expe- 

 rienced in those cases where they are obviously of a 

 very recent date, of which innumerable examples have 

 been pointed out in these islands; nor is it difficult to 

 conceive, that, through a period of infinitely longer dura- 

 tion, the greater number, or the whole of these rocks, 

 should have disappeared in those instances where their 

 formation has preceded that of the secondary rocks. The 

 peculiar circumstances which attend the overlying rocks 

 now connected with granite, such as those of Glenco and 

 the adjoining districts, of which the character is generally 

 porphyritic, although occasionally not distinguishable from 

 the more recent traps, indicate these as being in all pro- 

 bability the remains of trap deposits which may be 

 considered as of the primary class, and as the last remains 

 of masses once more extensive. That tract is not distant 

 from the spot under review; and under former circum- 

 stances, of greater extent, and of greater variety of com- 

 position, may have been the source of the nodules in 

 the conglomerate of Oban which has given rise to this 

 discussion. 



The last rock that remains to be described in Kerrera 

 is the trap, which, if not absolutely and always upper- 

 most, presents sufficient indications of its posteriority to 

 all the other rocks. Till a new classification of these 

 rocks is adopted, I am compelled to use this general 

 term; although it here presents varieties much in want 

 of more distinctive appellations. It is recognised at a 

 distance by the total absence of stratification, and by 

 the vertical fracture which its precipitous faces every 

 where display. 



As in other cases, the different modifications or mem- 

 bers are here found gradually changing their characters 

 and passing into each other. Thus greenstone passes into 

 basalt, or into clinkstone, or compact felspar, or into 

 porphyries and amygdaloids of various aspects. Many 



