42 GEOLOGY OF TUB DEBTEE BASIN. 



In the loess of the Denver Basin the evidence shows that the lower 

 part at least must have been of subaqueous origin. It seems necessary to 

 suppose that, after the rapid erosion which carved out the ancient river 

 beds, by some tilting of the plains region its general slope was so reduced 

 that the abundant water, resulting from the melting of the ice in the moun- 

 tains, gradually backed up in a temporary lake which existed long enough 

 for the settling to its bottom of this fine silt, which was readily carried a 

 long distance from its shore-line, and that subsequent tilting in a reversed 

 sense produced the present slope of the plains and the conditions of modern 

 drainage. It is quite possible that after the waters had entirely drained 

 away there would have been, under favoring climatic and meteorologic 

 conditions, a certain rearrangement of and adding to this material, which 

 would thus have been of subaerial origin, similar to that forming at the 

 present day in China. 



Before this subject can be satisfactorily treated, however, it is neces- 

 sarv to learn more than is at present known of the extent of this loessial 

 material. 



STRUCTURAL GEOLOGY. 



The tectonic or structural geology of an area like the present, though 

 at first glance apparently very simple, involves some of the radical prob- 

 lems of geotectonics or mountain building, and it is therefore important to 

 note all the details of its structure and endeavor to draw some conclusions 

 as to the manner in which the present structure was produced. It is not 

 proposed, however, to enter into any discussion of first causes — that is, 

 whether the forces which produced the observed deformations are the result 

 of the contraction of a cooling crust on a molten globe, of overloading of 

 areas near old shore-lines, or of any of the various hypothetical causes 

 which geologists and physicists have proposed to account for the observed 

 facts of the geological structure of our globe. Geologists are as yet far 

 from being in accord on these theoretical points, but those who have had 

 the most extensive field experience are agreed that the observed effects are 

 mosi readily accounted for by the action of intense forces of compression 

 of the outer crust of the globe acting generally in certain determined direc- 

 tions, whether these forces are the result of contraction or of any other 

 cause. 



