442 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 18 



of andesite. Later than all of these is a series of latite lava flows, which cap 

 the highest peaks of the district. 



The ore bodies are zones of fracturing and shearing in the Fraction rhyo- 

 lite breccia; strictly speaking they are lodes, not veins. The chief silver- 

 bearing mineral is cerargyrite (horn silver), which is commonly concentrated 

 in rich masses along irregular seams of sericite that traverse the lodes. The 

 primary metalliferous material is leanly mineralized rhyolite breccia carrying 

 a small amount of finely disseminated pyrite and threaded by thin veinlets 

 of exceedingly fine-grained quartz. What the primary silver-bearing mineral 

 is has not been determined. The scarcity of quartz or of silicification is a 

 noteworthy feature of the ores of the Divide district, especially in contrast 

 with the high silica content of the ores in the adjoining district of Tonopah. 

 The silver in the lean primary material of the lodes was concentrated by 

 downward enrichment as soft "sooty" argentite, and subsequently most of 

 this supergene argentite was converted to horn silver. A. K, 



GEOLOGY and HYDROLOGY.— ii;t;^/orator>' drilling for ivater and use of 

 ground water for irrigation in Steptoe Valley, Nevada. W. O. Clark 

 and C. W. RiddELL, with an introduction by O. B. Meinzer. U. S. 

 Geol. Survey Water-Supply Paper 467. Pp. 70, pis. 6, figs. 6. 1920. 



Steptoe Valley, in east-central Nevada, is about 95 miles long and 900 

 square miles in area. It lies in a structural trough between northward- 

 trending mountain ranges and contains thick deposits of alluvium with under- 

 lying lake beds. Ancient shore features show that the valley once contained 

 a lake about 30 miles long which discharged northward into Gosiute Valley. 

 The valley is too arid for agriculture without irrigation, and the supply of 

 surface water is meager. 



The paper gives the results of test drilling that was done b)^ the Geological 

 Survey, as authorized by Congress, for the purpose of finding water for irri- 

 gation. The test wells demonstrate that beneath the desert surface are 

 stored supplies of water adequate in quality and quantity for irrigation. 

 The paper also comprises a systematic description of the hydrology of the 

 valley with special reference to the interpretation of the results obtained in 

 drilling. It includes numerous measurements of stream flow and absorption, 

 on the basis of which it is estimated that the perennial streams contribute 

 about 20,000 acre-feet a year to the ground-water supply and that the total 

 annual ground-water supply is probably not less than 50,000 acre-feet. Cer- 

 tain plant species were found to feed habitually on the water in the zone of 

 saturation, and different species were found to indicate different depths to 

 the water table. The detailed map shows the zones of the principal water- 

 indicating plants, the areas of active and intermittent ground-water dis- 

 charge, and the estimated depths to the water table. It shows that water 

 is being discharged from the zone of saturation, through soil and plants, in 

 an area of about 115,000 acres. The paper also describes a number of large 

 springs, some of which are thermal. It includes a map of Nevada showing 

 38 Pleistocene lake beds thus far discovered lying wholly or partly in the 

 State, four of which were mapped in the present investigation. O. E. M. 



