PLEISTOCENE GEOLOGY. 259 



Platte, like that of the modern Platte, is almost entirely the detritus of 

 Archean rocks, and from the prevalence of pink orthoclase the gravel lias 

 a i-ikI(I\ tinge thai renders it easily distinguishable, even at a distance, 

 from the whitish sand of its southern affluents. The latter, especially 

 Plum ('reck, have contributed considerable quantities of rock fragments 

 characteristic of their respective basins, such as rhyoKtic tuff from Castle 

 Rock, cherts with poorly preserved Carboniferous fossils and Paleozoic red 

 sandstones from Perrys Park, silicified wood and the harder sandstone of 

 the Monument Creek beds, and smoky-quartz crystals from the granite of 

 the Front Range. 



The Platte drift is moreover distinguished from that of the smaller 

 streams l>v being more generally stratified, and its gravel is commonly cov- 

 ered with rusty stains, resulting from the decomposition of the iron sands, 

 and occasionally also with carbonaceous material, perhaps a remnant of 

 former vegetation. These stains are generally absenl from the drift of 



the smaller streams. 



The western tributaries of the older Platte, which also drained Archean 

 areas, contain only occasional fragments of basalt and audesite to distinguish 

 their drift from that of the Platte. They contain also placer gold in quan- 

 tities generally too minute for profitable working, but with considerable 

 accompaniment of heavy, black sands. The size of the constituents of the 

 drift varies in an inverse ratio with the distance which they have traveled. 

 Thus the coarser sands of Clear Creek are readily distinguished from those 

 of the Platte, and the latter again from the finer material of Plum, Cherry, 

 or Sand creeks. At Sterling, about 150 miles below 1 Denver, on the Platte, 

 the drift of both the ancient and modern streams is reduced to a coarse sand. 



The maximum observed thickness of the river drift, which is dependent 

 <>n the width of its bed and the slope of the bed rock, is 25 feet, and 1~> 

 feet may probably be taken as a fair average of that of the larger streams. 



Although modern erosion has removed a greater part of the mantle of 

 loess from the drift deposits of the ancient streams, enough still remains 



to partly obscure their outlines, and measure nts of the width of their 



beds can be only approximate. At Denver the attenuated western edge 

 of the Platte drift maybe noticed at many places along the western bank of 



