B6 GEOLOGY OF THE BLACK ITILLS. 



llio coarstr coiiglunicrates are .sometimes scarcely cemented at all, but are 

 loose a<r<ri-egations of water-worn fragments. 



A description of the character of the Potsdam sandstones of the Black 

 Hills could bo made in almost the same words that have been employed by 

 various observers in describing- the Potsdam in the eastern part of the 

 United States and in the liocky Mountains. Thus Professor Emmons 

 describes the Potsdam of northern New York as of two varieties : 



Ist. An even bedded and soniewluit porons rock, at many plsices a distinct white 

 i'liable .sandstone; in others it is a yelIowi.sh-brown sandstone, the particles of which 

 are compacted toy:ether so nH to form a Arm even-grained mass. 2d, A close-grained, 

 sliarpedged mass, • * * so do.sely wedged together that it is with difficulty 

 quarried. It is in fact a hard quartz rock [a quartzite] .scarcely passing for a sand- 

 stone.* 



And again, Professor Whitney, in the Report of the Geological Survey 

 of Wisconsin, says: 



As (h'veloped in tlie northwest and especially in the State of Wisconsin, where it 

 o(!cupies a large extent of surface, it is made up of an almost chemically pure silicious 

 sand in minute grains hardly larger than a pin's head, which are held together by the 

 minutest possible quantify of calcareous or ferruginous cement. Frequently even this 

 small quantity of cementing material is wanting, so that the rock can be readily crum- 

 bled between the fing(Hs like crystallized granular sugar. "Where the ferruginous 

 material, which is the peroxide of iron, becomes more abundant, so as to form two or 

 three per cent, of the mass, the sandstone acquires a dark brown color * * *.t 



Professor Comstock in the Wind River Mountains finds the Potsdam 

 to be a "succession of beds of loosely granular almost friable sandstone, 

 varying in color from red or brown beloiv to white above; in texture, from a 

 merely loose aggregation of the silicious particles to fairly compact sand- 

 stone."t IJis wonderful uniformity of the Potsdam will be seen by com- 

 ])aring these descriptions witli the following. 



In the Black Hills the Potsdam formation is a coarse-grained, loosely 

 granular, often saccharoidal sandstone, composed of small grains of quartz, 

 more or less rounded, ranging up to the size of a pin-head or small grain 

 of rice. Sometimes it contains also small scales of mica and grains of the 



• Natural History of New York. Geology. Emmons, p. 102. 



tOoologioal Survey of Wisronsin, IWVi. Vol. 1, p. 141. 



} Kecounaiiwauco of Northwi'st Wyomiug, etc. W. A. Jones, United States Engineers, p. 107. 



