THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. VIII. — The Igneous Origin of the " Glacial Deposits" 

 on the Navajo Reservation, Arizona and Utah ; by 

 Herbert E. Gregory. 



Introduction. 



The lowest recorded level reached by Pleistocene glaciers of 

 Utah and Arizona south of Lat. 40° is 8500,* and it was there- 

 fore a matter of considerable interest when glacial deposits 

 were reported from the Chinle Valley in northeastern Arizona 

 at an elevation of less than 5000 feet. 



The area in which the so-called glacial deposits occur is the 

 home of the pyrope garnets or "Arizona rubies" exported from 

 the Navajo Reservation, and a report by Sterrett on the pro- 

 duction of precious stones in the United States calls attention 

 to the unusual character of the material which overlies bed 

 rock in the garnet fields. 



"The drift is over 100 feet thick and is composed of bowlders 

 which vary from stones weighing many tons to cobble size, mixed 

 through a matrix of pebbles and sand. The gravel and bowlders 

 consist of biotite granite gneiss, porphyritic biotite granite gneiss, 

 hornblende or diorite gneiss, partly epidotized trap and basaltic 

 rocks, epidote hornstone, soapstone, tremolite asbestos, sugary 

 quartz, and large blocks of light gray colored fossiliferous lime- 

 stone of Carboniferous age. Just where the origin of this con- 

 glomeration is to be sought is not known. The general appear- 

 ance of the drift is that of a glacial deposit. Glaciation has taken 

 place in the San Francisco Mountains of Coconino County, 

 Arizona, and moraine deposits have been formed.f The latter 

 are thought to be of rather recent age, probably Quaternary. 

 Whether there has been glaciation in the slightly higher country 



* Dutton : Geology of the High Plateaus of Utah, 1880, p. 42. 

 t Atwood, W. W. : Glaciation of the Sau Francisco Mountains, Arizona, 

 Jour. Geol., vol. xiii, pp. 276-279, 1905. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 236. — August, 1915. 

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