208 



GEOLOGY. 



trations of the metamorphosing effects of igneous intrusions. The metamor- 

 phism is not contact metamorphism in the ordinary sense, but seems rather to 



fin. 



Fig. 82. — A section through Crow peak in the Black Hills region, showing the Black 

 Hills type of structure, pr, igneous rock (porphyry); €, Cambrian; S, Silurian 

 (Ordovician) ; CI and Cm, Carboniferous formations; Pm, Permian. Length of 

 section, 1| miles. (Jaggar, U. S. Geol. Surv.) 



combine cementation, injection, metasomatism, and mashing. 1 The date of the 

 intrusions is not known. Figs. 82 and 83 are sections of laccolithic hills in the 



72- P Cm CZ <S -C 



pJT 



Fig, 83. — Map of the region shown in section in Fig. 82. pr, porphyry; €, Cambrian; 

 S, Silurian (Ordovician); CI and Cm, Carboniferous formations; P, Permian; 

 Tr, Triassic. (Jaggar, U. S. Geol. Surv.) 



Black Hills region. They illustrate in a small way the structure of the Black 

 Hills. Fig. 83 is a map of the area of which Fig. 82 is a section. 



1 Van Hise, 16th Ann. Rept. U. S. Geol. Surv., Pt. I, p 814. 



