10 GEOLOGY OF THE DKNVKU BASIN. 



dunes constitute another topographical feature, distinct from any hitherto 

 mentioned. These are formed, as in other parts of the Rocky Mountain 

 region, on the wesl side of depressions broad enough to admit of a consid- 

 erable accumulation of wind-borne sand on the leeward side of the basin, 

 and on the slopes of divides too higb for the wind to carry them over. 

 Those within the area of the map, being not over 40 feet in height, come 

 within the interval of its contours, and are not outlined l>v them. 



The generally regular and gentle slope of the entire area is partic- 

 ularly favorable to its most important and permanent industry, namely, 

 agriculture under irrigation. Wherever there is sufficient supply of water, 

 it admits of its distribution from irrigation ditches over the areas below their 

 level with great uniformity and with a minimum expenditure of manual 

 labor. In consequence, the upland prairie country, which twenty years 

 ago was looked upon as practically valueless except as a grazing country, 

 is now covered with rich fields of grain and alfalfa, abounds in gardens, 

 ami constitutes one of the most valuable farming areas in the West. 



HISTORIC^ I. GEOIjOGY. 



PRE-CAMBRIAN FORMATIONS. 



'The present report has to do mainly with a comparatively recent phase 

 in the geological history of the Rocky Mountains. That considerable por- 

 tions of these mountains represent original land masses that have never 



been completely submerged, and that the sedimentary beds now resting on 

 their flanks and in part covering the crests of certain mountains have been 

 formed from the abrasion of these original land masses, has long ago been 

 demonstrated by the writer. 1 These original land masses, which consti- 

 tuted an archipelago of large islands in the Paleozoic seas, consist, so far 

 as determined by such examinations as have been made of the portions 

 now exposed, almost entirely of crystalline rocks, among which granites, 

 gneisses, and micaceous or hornhlcndie schists are the prevailing types. 



A distinct unconformity ami a pronounced change in lithological con- 

 stitution almost invariably mark the contact between this older crystalline 

 complex and the succeeding sedimentary beds of Paleozoic or later age, 



Second Ann. Rept.U. S. Geol. Survey, 1882, p. 211; Mon. U.S. Geol. Survey, Vol. XII, 1886, p. 20; 

 Bull. Geol. Soc. America, Vol. 1. 1890, p. 252. 



