South Dakota School of Mines 59 



land areas to the west, southwest, and northwest of their present 

 position, but from what earlier formations or from what kinds 

 of rock, except for certain local areas, has not been positively 

 determined. 



The coarser miaterials, particularly the heavy conglom- 

 erates and coarser sandstones between Rapid City and Buffalo 

 Gap, were clearly derived from the older rocks of the central 

 Black Hills, particularly the quartzites, granites, basic igneous 

 rocks, and vein quartz. 



Concerning- the finer deposits Xewton, in his report on the 

 geology of the Black Hills, 1880. says : "It is worthy of remark 

 that while the deposits of the Cretaceous, from the demolition 

 oi which the White River Tertiaries were evidently formed are 

 dark carbonaceous clays or shales, the latter are notable for 

 their light color. This is probably due to the making over of 

 the sediment in shallow water, whereby the carbonaceous matter 

 was oxidized, leaving the accumulation nearly white in color." 

 Hatcher, in his paper on the Titanotherium beds, 1893, states 

 that these beds were probably derived from two sources, namely, 

 from the Cretaceous clays and shales and from the kaolinization 

 of granitic feldspars. The sandstones he says are composed of 

 quartz, feldspar, and mica, and are evidently of granitic origin. 

 Matthew, 1901, considers that a considerable part of the 

 materials was perhaps derived from the Xiobrara chalk. 



Accepting the present generally accepted view as to the 

 manner of deposition of these deposits, a discussion of which has 

 just been given it is clear that there would be abundant op- 

 portunity for a thorough distributing and commingling of ma- 

 terial from many sources and for oxidation to the fullest extent 

 as suggested by Xewton. However that the Cretaceous rocks 

 were the main contributors may well need further proof in 

 view of the fact that many areas showing typical light colored 

 deposits lie far within the Cretaceous outcrops as we now 

 know them and in view of the further fact that the Tertiary 

 deposits were certainly at one time distributed over a far wider 

 region than was earlier supposed. 



GEOLOGIC HISTORY 



The rocks of the earth's crust retain to a marked extent a 

 record of their history. Sometimes this is indicated by compo- 

 sition, sometimes by manner of erosion, sometimes by relation 

 to one another, sometimes by fossil contents, et cetera. Often 

 several such characters are available in the same formation. In 



