2g6 



MATERIALS AND THEIR ARRANGEMENT 



ordinary shales cannot become basic schists, because in the produc- 

 tion of the muds from which shales are made, the bases were 

 mostly removed; but shales which are highly calcareous and magne- 

 sian may be changed basic schists (say hornblende schists) by meta- 

 morphism. Schists are commonly named for the abundant cleav- 



Fig. 287. Porphyry rendered schistose by pressure. Near Green Park, Cald- 

 well Co., N. C. (Keith, U. S. Geol. Surv.) 



able mineral constituent, as mica schist (chiefly of quartz and mica), 

 talc schist, chlorite schist, etc. 



The crystallizing processes of metamorphism are fundamentally 

 similar to the processes by which rocks crystallize from lavas; but 

 in metamorphism, the work is done chiefly by the aid of an aqueous 

 solution, while in the. solidification of lavas the crystallization is 

 from a mutual solution of the constituents in one another, where 

 water was but an incident. 



Metamorphic rocks are of course subject to deformation and 

 faulting, the same as sedimentary and igneous rocks. They are 

 also subject to alteration through decay, or through the reorganiza- 

 tion of their materials into new forms. 



