BLACK HILLS GEOLOGY. 



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trate, it has been either in the usual small thick-set intrusion, 

 such as that on Elk mountain or laccolitic masses, such as 

 Ragged Top, Crow Peak or the Needles. The latter are not 

 really intrusions in the Carboniferous, but were formerly cov- 

 ered by it and have been exposed by erosion. 



A glance at the sketch map, fig. 1 1, will bring out these rela- 



Fig. II. Sketch map showing relation of intrusions to fonnations into which 

 they have been intruded. The black portions are the eruptives, the white areas are 

 the Algonkian and Cambro-Silurian, the shaded areas are the Carboniferous lime- 

 stone. 



tions. The shaded areas are those covered by the Carbon- 

 iferous and overlying formations. The unshaded portions are 

 those from which the limestone has been completely removed. 



