272 GEOLOGY OF THE DEXVEE BASIN. 



They are of particular interest from a geological standpoint at the 

 present time on account of the interest that attaches to the South African 

 conglomerates, which have been supposed by some to be fossil placers. 

 The material of which the Monument Creek beds here consist is chiefly 

 granite, granite-gneiss, and quartzite, with some pegmatitic quartz; in other 

 words, material in which gold is known to occur in the adjoining- Colorado 

 Range. South and west of the heads of Newlin and adjoining gulches 

 these undisturbed beds extend in an apparently unbroken mesa as far as 

 the eye can reach. Tests made of the disintegrated material on the top of 

 the mesa show a small but fairly uniform content of gold in the beds. From 

 these facts and from the character of the auriferous gravels it seems evident 

 that the gold was not derived directly from the mountains, but is a concen- 

 tration of that which had been carried out in the waters of the lake to a 

 distance of at least 15 miles from its shore. The gold in the placers is 

 generally well rounded and flattened, and in quite small particles, the largest 

 observed being from 2 to -1 millimeters in diameter and less than half a 

 millimeter thick. 



WATER. 



Owing to its porosity the river drift readily absorbs large amounts of 

 the water coming from rain and seepage from irrigation canals, and wells 

 sunk to it often yield excellent water. 



In the smaller towns this quality of the beds obviates for a time the 

 necessity of constructing sewers, but the water in wells thereby becomes 

 polluted and with an increase of population the gradual accumulation of 

 decaying organic matter is liable to generate disease. 



A farm situated on river drift will require a greater quantity of water 

 for irrigating purposes than one situated on the more impervious loess, and 

 this difference may limit its profitable cultivation in dry seasons. 



The loess of the Denver Basin forms a soil that, though lacking in 

 organic matter, needs only the vivifying action of water to produce large 

 and frequent crops. It is thus peculiarly well adapted for a region of 



