154 



GEOLOGY. 



By way of summary it may be said that the system consists in some 

 places of rocks which are mainly massive (igneous); in other places, 



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Fig. 47. — Section showing the relations of Archean to younger formations in the 

 Crested Butte region of Colorado. JR, Archean; € Cambrian (Sawatch quartz- 

 ite); 0, Ordovician (Yule limestone); M, Mississippian (Leadville limestone); 

 Cw and Cm, Carboniferous (Weber and Maroon formations, respectively); J, Ju- 

 rassic (Gunnison formation); Kd, Kb, Kn, and Km, Cretaceous (Dakota, Benton, 

 Niobrara, and Montana, respectively) ; Erh, Eocene rhyolite. Length of section 

 6 miles. (Eldridge, U. S. Geol. Surv.) 



A rocks which are mainly gneissic (chiefly meta-igneous) ; and in 

 still others, of rocks (largely meta-igneous and subordinately meta- 

 sedimentary) in which a schistose structure predominates. Furthermore, 

 the rocks of each of these structural types have a wide range in com- 

 position, from acid on the one hand to basic on the other. Rocks 

 of all these classes are often intimately associated, and any one may pre- 

 dominate over the others to varying extents. In some places the rocks 

 of these several structural types graduate into one another so completely 

 as to leave no line of separation, while in others their definition is 

 sharp. Thus massive rock sometimes appears in well-defined dikes 

 which cut the gneisses and schists at any angle, while on the other hand, 

 schists are frequently in dike-like sheets in rocks which are more mas- 

 sive. In the first of these cases, the gneissic or schistose rock which 

 incloses the dikes of massive rock probably suffered metamorphism 

 before the intrusion of the latter, which has never been changed; in 



Fig. 48. — Section showing the relation of the Archean to younger formations in the 

 Ten Mile district of Colorado. Ai, Archean; C, Carboniferous; J, Jurassic; 

 emp, Elk Mountain porphvry, Length of section, 4 miles. (Emmons, U. S. 

 Geol. Surv.) 



the second, the inclosing rock was not metamorphosed before the 

 intrusion, and in the changes of subsequent time the intrusive rock 



