178 



GEOLOGY. 



on the character of the rocks, often rendering them metamorphic. Since 

 the metamorphism is, on the whole, most complete where the deforma- 

 tion of the beds has been greatest, it is inferred that dynamic action 

 was a principal factor in the metamorphism. As a result of the meta- 

 morphic change, slates and schistose rocks of various sorts came to be 

 important components of the system. Fig. 63 represents, about natu- 

 ral size, the structure of a piece of Huronian rock after deformation. 

 It is not to be understood, however, that dynamic and thermal 

 agents were the only ones involved in the changes which the Huronian 







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Fig. 63. — Figure showing the deformation of a small piece of Huronian rock. The 

 dark bands are jaspilite. The remainder is iron ore (hematite). The character of 

 the bending and breaking shows that the deformation took place in the zone of 

 combined fracture and flowage. (Van Hise, Bayley, and Smyth, Mono. XXVIII, 

 U. S. Geol. Surv.) 



formations have suffered. The sandstones and the quartzose conglomer- 

 ates have been transformed into quartzites by other means (p. 170), and 

 the iron-bearing formations, originally laid down as cherty ferru- 

 ginous carbonates in some cases, and as ferrous-silicate-bearing sedi- 

 ments in others, have been notably changed by percolating waters 

 carrying Oxygen, carbonic acid, etc. One of the results of this 



