116 REPORT UNITED STATES GEOLOGICAL SURVEY. 



the close of the latest volcanic eruptions. It is possible, therefore, that 

 it may have reached into the historical period. According to Major 

 Powell some of the Pai-ute tribes have legends indicating volcanic out- 

 flows, and, as has often been suggested, many of our western basalts 

 have a very "fresh" appearance. 



Glaciers have not wrought any radical changes in the general config- 

 uration of the country. They have modified certain features, have deep- 

 ened certain portions, levelled others, but they have not, alone, carved 

 deep canons, or carried away hills and ridges, leaving in their stead 

 level valleys. Much of the drift that today we regard as "river-drift" 

 was undoubtedly first removed from the original place of deposition by 

 glacial action. Silt and soil both can be formed bythe never-ceasing 

 action of moving ice and water upon rocky material. Carried on by the 

 water these were deposited near the moraines, until eventually once 

 more they were washed away to form soil for arable lands. 



DRIFT. 



A number of drift- varieties will always be found in a region so diver- 

 sified as the State of Colorado. We can distinguish, mainly, glacial 

 drift, lake-beds, river-drifts, and avalanchial drift. Of these the first is 

 simply the morainal accumulations. As soon as these have been removed 

 they lose their identity. Lake-beds are the accumulations of finely sepa- 

 rated drift in a body of still water. Frequently such drift may be ob- 

 served occurring in broad valleys, where eventually either the gradual 

 rise of the lake-bottom or changes of niveau have permitted the water 

 to flow off. Eiver-drift is the species most frequently met with. That 

 tendency of flowing water, ever to straighten its course, causes it to 

 cover, in time, often a valley of considerable extent. I call this " parallel 

 shifting of rivers." Thus, frequently, an entire valley may be covered by 

 river-drift. Erroneously, this fact has often been explained by the 

 assumption that at some former period the stream was one of far greater 

 breadth than at the present time. Though this is certainly true in some 

 instances, it is a very rare case. Accumulations of drift on one or the 

 other bank of the stream will, locally, change its course. More material 

 will be deposited on that same side ; the river will be shifting away from 

 it. Gradually it may have traversed the entire width of the valley in 

 this manner, leaving evidence of its former presence in the drift it has 

 deposited. This feature can admirably be studied near the junctions of 

 large streams. 



By the term "avalanchial drift" we designate the ever-moving recent 

 deposits of rock-fragments on the sides or at the base of mountains. 

 Constantly the rocks composing mountains and peaks are disintegrating 

 and rolling down from a more or less loosely joined talus of enormous 

 dimensions. With the character of the rocks composing it changes the 

 nature of the talus. The harder and more angular the fragments the 

 less stable the slope. Decomposition attacks several kinds of rocks very 

 readily, and then a stratum of soil is formed on the talus that permits the 

 growth of vegetation. 



SOIL. 



The eventual result of disintegration and partial decomposition of 

 rocks is the formation of soil. A very large portion of Colorado is too 

 high for agricultural purposes, so that a majority of the best, most i^ro- 

 ductive soil must necessarily always remain idle.' In other localities, 

 where altitudinal conditions are more favorable, the want of an adequate 



