276 GEOLOGY OF THE DENVER BASIN. 



as thickest on the high prairies between the main water courses, where wells 

 are sunk through it to depths of 100, 150, and 180 feet in order to reach 

 the water-bearing stratum (Tertiary grit) below. From the descriptions 

 given by the above-named observers there would appear to be a very close 

 correspondence in physical characteristics between the loess of Kansas and 

 Nebraska and that of eastern Colorado, but no microscopic or trustworthy 

 chemical examinations 1 appear to have been made of the material from 

 either of those States. 



As compared with the loess of the Mississippi Valley, the loess of 

 Colorado would appear to lie somewhat coarser, though data are wanting 

 for any definite comparison. The greater part of the former, up to 80 or 

 90 per cent according to Chamberlin and Salisbury, is not more than 

 0.0025 mm. in diameter, while the coarser sand, which constitutes a consid- 

 erable through not a definitely known proportion of the Colorado loess, lias 

 a "-rain between 0.1 and 1 mm. in diameter. The grain of the Colorado 

 loess, moreover, appears to decrease in size as the distance from the 

 mountains increases. 



The chemical composition of a mechanical mixture like the loess is 

 not an absolute means of correlation between widely separated bodies of 

 material, but is rather useful in indicating local differences of origin in 

 the same general region. Still, a comparison of the analysis of material 

 from the two localities discloses nothing inconsistent with similarity of 

 origin. 



The Colorado loess exhibits an appearance of stratification that in it 

 lower portions, near the waterways, is very marked, which is in favor 

 of a subaqueous deposition. A still stronger argument in this direction is 

 afforded by the glacio-natant till, which can best be accounted for as having 

 been dropped from floating ice while the sediments were still in a sufficiently 

 incoherent condition to admit of the fragments sinking through to or near 

 their base. The fact that the deposits extend up only to a given level, and 

 an- wanting above that level, is probably the strongest argument against 

 the seolian and in favor of the aqueous theory. 



1 Aughey's report gives five analyses of loess from different parts of Nebraska, but these bear 

 such internal evidence of having been either badly made or incorrectly reported that no reliance is 

 placed upon them. 



