178 SCARBA. GEOLOGY. 



since, in the more minute and sudden contortions, although 

 the schist has yielded so as to conform itself to the new 

 position into which it was forced, the quartz rock has 

 been fractured ; and instead of presenting contortions 

 and flexures parallel with those of the schist, is irregu- 

 larly intermixed with it, or deviates from the parallel and 

 alternate position which it maintained in the former case. 

 Had such contortions been the effect of crystallization, 

 it is probable that the compound mass would have obeyed 

 the same laws in the less as in the greater instances ; and 

 that, in both cases, there would have appeared the same 

 flexure, elongation, and parallel adaptation of parts in the 

 quartz rock, and in the schist. 



This summary of the geological constitution of Scarba 

 includes the principal appearances ; and, in omitting 

 minor details, I have been influenced by the recollection 

 that similar facts are described either in one or another of 

 the associated islands. 



But I must not terminate this account without offering 

 some remarks on the alternations of these schistose rocks ; 

 since their order in these islands contradicts some pre- 

 valent opinions, and leads to important results both in 

 geological investigations and in the formation of a general 

 theory. It will be proved in treating of Jura, that quartz 

 rock alternates with and graduates into micaceous schist, 

 and that they are consequently as much members of one 

 deposit, as common sandstone and shale. Their geologi- 

 cal place is therefore the same. It is generally said that 

 clay slate is the next rock in order to these, and that it 

 follows micaceous schist as \b is itself followed by gray- 

 wacke. But I have on various occasions shown that 

 graywacke and clay slate alternate. There is conse- 

 quently, in these two rocks, an equivocal relative position 

 to the surrounding strata, similar to that of micaceous 

 schist and quartz rock, while each pair presents a certain 

 analogy of composition to the other. As the finer and 

 more easily suspended substances which constitute mica- 



