274 GEOLOGY OF THE DENVER BASIN. 



BRICK CLAYS. 



The loess affords the greater portion of the brick earth used in the 

 State, and, although somewhat deficient in argillaceous constituents, makes 

 an ordinary brick that answers the requirements of the climate. The 

 alluvium of the streams is also used for the same purpose. 



Small portions of gold in the silt have become concentrated on the 

 sin lace of the sand rock beneath the loess, but it has not been found in 

 sufficient abundance for profitable extraction. 



ORIGIN OF THE LOESS. 



It was not practicable in the course of the present investigation to 

 make such an exhaustive study of the material that has here been given 

 this name as would definitely correlate it with similar material in the 

 Mississippi Valley or throw any new light upon the general question of 

 the origin of loess. Still, it may lie well to discuss briefly the facts that 

 have been determined which bear upon this question. 



The origin of loess has long been the object of much speculation 

 among geologists, and various theories have been advanced to account for it. 

 for a longtime the theory most generally received in Europe, where it was 

 first observed and studied, was that it is glacial silt, accumulated in 

 temporary and somewhat ill -defined basins, but difficulty has always existed 

 in accounting satisfactorily for the inclosing of these basins, on account of 

 the peculiar positions, in reference to the present topographical configuration 

 of the region, in which the loess is sometimes found. 



Von Richthofen's study of the great loess deposits of northwestern 

 China and his demonstration that they are the accumulation of wind-trans- 

 ported material from the great steppes of the interior of Asia, produced a 

 great change in theoretical views of geologists, and a probable seolian 

 origin was adopted by many for the loess of Europe and even of the 

 Mississippi Valley. 



The geologists who of late years have made studies of the loess of the 

 Mississippi Valley generally tend, on the other hand, to return to a glacial 

 origin for the loess of that region. The latest writer 1 on the subject 



1 W. .1. MoGee, Pleistocene history of northeast Iowa: Eleventh Ann. Kept. U. S. Geol. Survey, 

 1881, p. 302. 



