HYDROLOGIC CONDITIONS ' 2l7 



took place in very recent time, an inference which agrees well with the 

 supposition that the Afton craters were formed during the second or 

 latest period of volcanic activity. 



Hydrologio Conditions 



The valley filling is saturated with water' below a depth of 100 feet 

 beneath the bottom of the Kilburn crater, a level nearly corresponding 

 with that of the river, as shown by wells sunk in various parts of La 

 Mesa. The water obtained in the Kilburn crater is charged with hydro- 

 gen sulphide and has a temperature of nearly 100° Fahrenheit, 15° or 

 more warmer than water from the same horizon in surrounding wells. 



Other Depressions similar to the Afton Craters 



Depressions resembling more or less closely the Afton craters have 

 been described from several localities. Among those comparable in size 

 with the Afton craters and most closely resembling them in character, 

 two may be mentioned: Coon butte, near Flagstaff, Arizona, described 

 by Gilbert* and others, which is a circular depression about 3,800 feet in 

 diameter and 400 feet deep, surrounded by a crater rim 160 feet high, 

 and Zuni Salt lake, in western New Mexico, described by Dartonf and 

 others, which is a nearly circular depression about a mile in diameter 

 and 350 feet deep. 



Comparable also, in some measure at least, are such volcanic phenom- 

 ena as calderas like that of Crater lake in Oregon, J and the crater rings 

 described by Russell§ from the Snake Eiver region of Idaho, in which 

 two depressions occur, the larger being 1,100 feet across and 200 feet 

 deep. 



Among other depressions which may be compared with the Afton 

 craters are Montezumas well, || in Arizona, a circular depression about 600 

 feet in diameter and 170 feet deep, and Salt well,^ in southern Nevada, 

 300 feet in diameter and 65 feet deep. 



* G. K. Gilbert : Science, new series, vol. 3, 1896, p. 1. 



t N. H. Darton : The Zuni Salt lake. Journal of Geology, vol. 13, 1905, pp. 185-193. 



+ J. S. Diller and H. B. Patton : The geology and petrography of Crater Lake National 

 park. Professional Paper no. 3, tJ. S. Geol. Survey, 1902. 



§ I. C. Russell : Geology and water resources of the Snake River plains of Idaho. 

 Bull. no. 199, V. S. Geological Survey, p. 110. 



II William P. Blake : Origin of the depression known as Montezumas well, Arizona. 

 Science, new series, vol. 24, 1906, p. 568. 



H G. K. Gilbert : Wheeler Survey, Geog. and Geol. Survey West of 100th Meridian, 

 vol. 3, Geology, 1875, p. 109. 



