abstracts: geology 65 



tion to local flexures of small dimensions, were noted in the region 

 between the San Juan and the Puerco and Little Colorado. 



The physiographic history of the Navajo country is included in that 

 of the Colorado Plateau. The stratigraphic series of the area is essen- 

 tially that of the Grand Canyon district; the crustal movements of 

 .the two areas, though different in kind, were probably contempo- 

 raneous, and it is reasonable to suppose also that the periods of igneous 

 activity for the whole Plateau province are closely related in time. 



The surface of the Navajo country has been carved rather than 

 built; features resulting from deposition are I'elatively unimportant. 

 Talus slopes and alluvial fans are replaced by cliffs; hills and knolls 

 give way to buttes and towers; and graded slopes are represented by 

 walls sculptured into rincons, recesses, alcoves, niches, windows, and 

 arches, of large variety. 



Wide, open mouths of niches and caves perched high on the canyon 

 walls are conspicuous. These cavities, protected from rain, from the 

 glaring heat of the sun, and from the suffocating sandstorms, were 

 widel}^ utilized by the ancient inhabitants as building sites — ''rock 

 shelters" or "cavate dwelhngs." 



Perhaps the most striking erosion feature within the Navajo coun- 

 tr}' is the recently discovered Rainbow Bridge, which spans Bridge 

 Canyon, on the northwest slope of Navajo Mountain. Its symmetry 

 and graceful proportions, as well as its size and beauty of color, give 

 to this arch a commanding position among the natural bridges of the 

 woi'ld. . R W. Stone. 



» 



GEOLOGY. — Ground water for irrigation in Lodgepole ]'alley, Wyoming 

 and Neh'aska. Oscar E. Meinzer. U. S. Geological Survey 

 Water-Supply Paper 400-B. Pp. 33, with 4 maps. 1917. 

 This brief report discusses the distribution and water-bearing char- 

 acter of the Ogalalla and Arikaree formations, the Brule clsiy, the 

 underlying formations, and the Quaternary stream gravels. Large 

 yields are obtained in the valley from the stream gravels and from the 

 underlying jointed portions of the Brule clay. The strongest well 

 3nelded, in a test of one hour, at the rate of 710 gallons per minute with 

 a drawdown of less than 4 inches. The report describes the natural 

 processes of storage of water in the underground reservoir and of dis- 

 charge therefrom by gravity in the irrigation season. It also gives 

 data by the author and by Mr. H. C. Diesem, of the Department of 

 Agriculture, on the cost of pumping for irrigation. 0. E. M. 



