102 REPORT UNITED STATES GEOLOGICAL SURVEY. 



interstratum, and the succeeding frosts and thaws had eventually prO" 

 dueed the arch. 



Four kinds of "monuments" may be distinguished. More frequently 

 met with than any others are those composed entirely of sandstone. 

 Then follows the form having a pedestal of shale and a sandstone cap. 

 Monuments eroded from compact conglomerate are frequent in certain 

 localities. One class of occurrences might be termed " accidental" mon- 

 uments. This last one is composed of some eroded material, capped by 

 a single erratic bowlder, which has its origin sometimes far distant. 

 Aqueous erosion may be considered as the primary impulse toward the 

 formation of sach monuments. Detail features are subsequently pro- 

 duced by atmospheric agents, and by sand driven before the wind. All 

 of these forms depend upon having a hard, protecting cap, which will^ 

 at least for some time, prevent a disintegration and eventual removal 

 of the more or less regular column supporting it. A great deal has 

 been said and written about the famous groups of Colorado, belonging 

 to this category, so that this brief allusion to their existence must suf- 

 fice. By their peculiar and picturesque appearance they attract the 

 eye of even a passing traveller, and have, therefore, become justly re- 

 nowned. 



DRIFT. 



Water, flowing, has the most pronounced action upon drift. In dis- 

 cussing the latter, avalanchial drift will not be considered, but only that 

 deposited directly by ice or water. So far as the result of erosiou is 

 concerned the effect upon the two is essentially the same. Both precipi- 

 tated and flowing water have a tendency to remove from the drift the 

 smaller portions of detritus and the slight accumulations of clay and 

 sand and silt. Thus the final result would be an accumulation of erratic 

 bowlders, without any sand or clay connecting the single pebbles. As 

 a rule this final result is not achieved, however, because the same water 

 that removes sand and silt from one locality deposits it within the drift- 

 area at another. An accumulation of the smaller drift particles forms 

 alluvial soil. Here, too, the endeavor is constantly noticeable to remove 

 finer, lighter particles and leave only the heavier ones. Were it not for 

 the unceasing supply from other sources, our alluvial soil would even- 

 tually disappear as such. Deep ravines and gorges are cut into th^ 

 drift by flowing water, and large quantities of it are thereby trans- 

 ported to other localities. 



All rocks, upon disintegration and decomposition, partial or complete, 

 form soil, and, dependent upon the lithological constitution of the origi- 

 nal rock, this soil may be classified. 



CONCLUSION 



With the above report the discussion of my field-work in Colorado is 

 completed. From 1873 to 187C, inclusive, I was engaged in studying 

 the geological features of that State at various localities. During that 

 time about 27,000 square miles were surveyed by the parties to which I 

 was attached. By an excellent arrangement of the field-parties, I was 

 enabled to explore contiguous areas during the three first years. Their 

 extent amounted to about 23,200 square miles. It is evident that thus 

 the work was greatly facilitated. By being able to trace the same forma- 

 tions over so large an area many points in question were cleared up 

 that otherwise must have remained doubtful. Correlations of forma- 

 tions and groups appeared in a more definite light, and were more 



