428 W. li. TWENIIOIEL — -THE SILVER CITY QI ART2ITES 



(5) N'ot one of the altered rocks has had developed any parallel orien- 

 tation of its minerals or an}^ structure which can be referred to dynamic 

 metamorphism. 



Causes of the Alteration 



It is quite possible for quartzite to be developed through processes of 

 ordinary cementation of sedimentary rocks. IsTo special conditions are 

 required for the development of clicrt and flinty since these are known to 

 be commonly formed in the zone of fracture in the presence of cold water. 

 Tlie occurrence of tliese rocks, tlierefore, requires nothing other than ordi- 

 nary sedimentary and ground-water conditions. No known process of 

 sedimentation, liowever, can account for the develo]:)ment of the horn- 

 blende and its origin must be referred to some other process, and as the 

 chert and quartzite appear to l)e genetically related to the hornblende, it 

 is inferred that all three, together with tlie chlorite and the epidote, 

 originated in the same manner. 



It has been pointed out that tbere is no parallel development of min- 

 erals in any of the rocks, so that pressure metamorphism as a cause of 

 the alteration is eliminated. ]\retamorphism of the contact type appears 

 to be out of question, as no igneous rocks have been recognized in the 

 strata altered or contiguous thereto. Furthermore, the alteration is quite 

 unlike that generally associated with heat Tn^tamorph ism. This leaves 

 hydrothermal metamorphism as the only possible cause of the alteration, 

 and everything which was observed is in harmony with the view that tbe 

 quartzites and associated rocks were developed through the circulation of 

 hot waters. Mudge explained the metamorphism in the same manner.' 

 He, however, failed to observe the hornblende, epidote, and chlorite. 



The sequence of events in the Silver City area is believed to have been 

 something as follows: Prior to the deformation of the strata a quite nor- 

 mal lithology of the three divisions known to have been modified obtained 

 in the Silver City area. Deformation locally l)recciated the upper latan 

 limestone — the only brittle member of tlie known modified rocks — and 

 opened cracks to gi'eater depths. (Circulating waters which were probably 

 of surface origin removed the soluble constituents from the fragments of 

 the limestone, leaving the clay and silica with perhaps some increase of 

 the latter. Later, hot solutions coming from great depths circulated 

 through the fractured and porous zoiies and idtimately filled the pores 

 and cracks with chert, hornblende, chlorite, and epidote and cemented 

 the fragments togetber with substances of the same character. The sand- 



Mndge : Loc. cUod, p. 12. 



