122 HATCHER — OLIGOCENE AND MIOCENE DEPOSITS. [Aprils, 



horizons, show frequent examples of cross-bedding and their irregu- 

 lar course, as well as the spheroidal shape of the pebbles they con- 

 tain/ and the increased fineness of the materials of which they are 

 composed as one proceeds from the margin toward the interior, are 

 all characters strongly suggesting that they were deposited in river 

 channels. Moreover, the materials of these sandstone and con- 

 glomerate lenses are not only coarser about the western borders of 

 the beds, but the lenses are far more numerous in that region. 

 Toward the interior these lenses converge and unite without spread- 

 ing out laterally, so that in the region lying east of the Black Hills 

 in South Dakota, at a distance of fifty to seventy-five miles from 

 the mountains, the sandstones are finer, less frequent and are sepa- 

 rated by greater areas of fine clays, just as the streams of the present 

 day unite and become fewer in number as we proceed farther from 

 their sources. 



The Metamynodon and Protoceras sandstones, as well as certain 

 intermediate and underlying sandstones, present many evidences, 

 like those just enumerated, which strongly suggest that they were 

 deposited in river channels. Taking the Protoceras sandstones as 

 the most favorable example, owing to the greater extent to which 

 they have been exposed by the subsequent erosion of the overlying 

 sediments, they are seen to extend as a series of narrow elongated 

 lenses from the summit of the Cheyenne and White River divide 

 for several miles to the southward of the last-mentioned stream, 

 where they pass beneath more recent deposits. Throughout their 

 entire extent they exhibit frequent examples of cross-bedding, 

 while the sands become finer and the channels fewer in number and 

 broader and deeper as ones goes southward toward and across 

 White River. That they have been removed by erosion over con- 

 siderable areas lying between their present limits and the Black 

 Hills is evident. At the summit of the Cheyenne and White River 

 divide there are several of these sandstone lenses at approximately 

 the same horizon. These bear many evidences of having been 

 deposited in the channels of small streams or rivers pertaining to a 

 single drainage system, which had its source somewhere in the 



^A conglomerate accumulated by a running stream can usually be distin- 

 guished very readily from one formed on the beach of a lake or sea, by the shape 

 of the contained pebbles. In the first instance the pebbles have been reduced to 

 irregular spheroids by the rolling motion to which they have been submitted by 

 the current. In the second they are more generally flattened disks. 



