BLACK HILLS GEOLOGY. 201 



In general little can be added to the work of Newton as re- 

 gards the Cambrian. There are, however, several points de- 

 serving of special mention. The first is that the Cambro- 

 Silurian formation as a whole is one which, from its thinly bedded 

 character, and the easily cleavable nature of its component 

 sediments, has afforded an extremely easy access both to the 

 intrusion of igneous rocks, and to the passage of ore-bearing 

 solutions ; the second is that the high percentage of carbonate 

 of lime, which characterizes most of its shales and sandstones, 

 has rendered them very susceptible to replacement and thus en- 

 abled siliceous solutions to deposit their burden of silver and 

 gold. 



3. Carboniferous. 



The Carboniferous formation is represented in this district by 

 a series of very heavily bedded gray limestones, which attain a 

 considerable thickness to the west of Spearfish creek. They 

 dip with the underlying Cambro-Silurian strata in a westerly 

 direction and gradually become thicker as we pass from the 

 eastern border of the exposure, until in Spearfish carion they 

 attain a thickness of over 500 feet. 



No careful section of this formation was made, but the impor- 

 tant features of the series are its great thickness and its massive, 

 homogeneous character. The latter has made it an extremely 

 resistant rock, both to the intrusions of igneous magmas and 

 to the passage of ore-bearing solutions. Shaly horizons oc- 

 cur only at rare intervals, and then are separated by consider- 

 able thicknesses of more massive strata. 



The relation of the Carboniferous formation to the topography 

 has already been touched upon. The character of rock is re- 

 sponsible for the deeply carved and precipitous nature of the 

 gulches. The streams which have penetrated the limestone 

 have found the underlying shales of the Silurian formation an 

 exceedingly soft and easily eroded series, and have cut them 

 from beneath the heavy limestone above, causing the latter to 

 b'-eak off in huge blocks, that have left behind great perpen- 

 dicular cliffs and have strewn the bottom of the gulches with a 

 Annals N. Y. Acad. Sci., XII, Nov. 23, 1899 — 13. 



