THE DECOMrOSITION OF THE EOCKS. 



73 



jected to decomposing influences not shared by the other rocks in the same 

 degree. It is diflicult to avoid the conchision that tlie jieriod of intense 

 chemical action cannot antedate the eruption of Later hornblende-andesite, 

 and probably succeeded it. 



Fig. 1. — Area of extreme decomposition. 



Not only have tlie same minerals in tlie ^'arious rocks undergone iden- 

 tical processes of alteration, but similar groups of minerals have yielded 

 almost identical results. Hornblende, augite, and mica have given place to 

 the same ultimate products, though in slightly different proportions, and the 



