THE ORIGIN AND DESCENT OF ROCKS. 449 



as a degradational process, katamorphism. In both cases, however, 

 there is solution and rearrangement of the molecules. The katamor- 

 phism of basalts and other basic rocks gives basic schists; that of 

 granitic and similar rocks gives gneisses. The anamorphism of basic 

 pyroclastic tuffs and wackes gives basic schists, while that of acid pyro- 

 clastics and most shales gives gneisses, mica schists, or similar acidic 

 schists. It is obvious that ordinary shales cannot usually become basic 

 schists, because in producing the original muds, the bases were generally 

 removed; but when shales are highly calcareous and magnesian, as 

 when they grade toward the limestones and dolomites, they may become 

 basic schists by metamorphism, e.g., certain hornblendic schists. It is 

 even more obvious that the limestone and sandstone formations must 

 largely retain their distinct composition. It is thus seen that, in general, 

 a sedimentary series anamorphosed must differ from a crystalline series 

 katamorphosed, though both give rise to foliated or schistose rocks. 



Deep-seated metamorphism. — When the exceptional pressure arises 

 from the weight of rocks felt at great depth, it is practically equal in all 

 directions and the crystalUzation probably develops normally and is not 

 forced into the parallel or foliated form. Rocks metamorphosed under 

 these conditions probably tend to take the massive form rather than 

 the schistose form, but this conclusion is theoretical rather than ob- 

 servational, for little or nothing is known of the history of such rocks. 



Completion of the rock cycle. — The crystaUizing processes of meta- 

 morphism are fundamentally similar to the processes by which rocks 

 crystallize out of magmas, only in the first case the work is done chiefly 

 by the aid of an aqueous solution, while in the second it is done through 

 a mutual solution of the constituents in themselves, where water was 

 but an incident. If the heat factor in metamorphism be sufficiently 

 increased, aqueous solution may actuaUy grade into magmatic solution 

 through various degrees of softening and melting, and the cycle of 

 changes be closed in upon itself. 



VARIOUS CLASSIFICATIONS AND NOMENCLATURES. 



From the foregoing sketch of the processes of rock-making it 

 may easily be inferred that the varieties of rocks may be almost un- 

 limited, and that they may be defined, named, and classified on many 

 different bases ; for example. 



(1) If the mode of origin is chiefly in mind, rocks may be classed as 



