262 GEOLOGY OF THE DENVEB BASIN. 



indications of some changes in chemical composition. Thus the lower por- 

 tions, where exposed, contain frequent white, pulverulent spots, due to the 

 infiltration and concentration of carbonate of lime from the upper beds, 

 which sometimes, though rarely, develop into the mannchen tonus found in 

 the Rhine loess. A general decrease in argillaceous material as the distance 

 from the mountains increases is shown in loss of plasticity and decreasing 

 ability of the loess to maintain vertical faces in artificial excavations. The 

 paler color of the loess and of the bricks made from it indicates also a 

 decreasing proportion of ferric constituents. 



Chemical and microscopical examinations were made of a few 

 characteristic specimens of loess and soil from different parts of the region 



under consideration. These confirm in general the above indications, and 

 show, moreover, that the fineness of grain and the degree of decomposition 

 of the component part- of the material are also proportional to the distance 

 from its source. It contains in all cases a large proportion of tine sand, 

 separable In washing, whose grains are generally under a millimeter, hut 

 rarely less than a tenth of ;i millimeter, in diameter. 



In the loess from North Denver a rough calculation showed that this 

 sand contains about 40 per cent of quartz, 50 per cent of feldspar, and 10 

 per cent of other constituents. The latter include white mica and mag- 

 netite, with augite and hornblende. The feldspars are more or less decom- 

 posed, and the quartz grains are sometimes rounded, sometimes angular. 

 Some of the material taken at 20 feet from the surface is recognizable as 

 eruptive and probably derived from the Denver beds. This loess is friable, 

 crumbling easily in the fingers, and appears to contain no cementing mate- 

 rial, as is continued by the absence of carbonates in its analysis. 



The loess from eastern Colorado, near Wray, is a fine-grained, slightly 

 coherent sand, with a coating of carbonate of lime covering the grains and 

 acting as cement. The sand itself contains the same constituents as the 

 Denver loess, hut the quartz is in larger proportion. At some distance' from 

 the surface it is more coherent from its greater proportion of carbonates, 

 and the concretions contain sufficient lime to make nearly NO per cent of 

 carbonate if it is all in that form. 



The loess-like earth from ( 'hevenne contains a greater variety of min- 

 erals among its constituents than any of the samples examined, and a larger 



