W. B. Emery — Igneous Geology of Carrizo Mountain. 319 



Art. XXXVII. — The Igneous Geology of Carrizo Mountain, 

 Arizona ;* by Wilson B. Emery. 



During the Summer and Fall of 1913, while employed as 

 a field assistant by the United States Geological Survey, the 

 writer bad the opportunity of carrying on, under the direction 

 of Professor Herbert E. Gregory, the first detailed geologic 

 investigation ever made of Carrizo Mountain, Arizona. Recon- 

 naissance studies had been previously made by W. H. Holmes 

 in 1875f and in 1909 by Professor Gregory in connection with 

 his work on the Navajo Reservation. ;{: During his brief 

 sojourn in the area Professor Gregory noted the main features 

 of the geology and it was because he thought them of sufficient 

 importance to repay detailed examination that the work during 

 the season of 1913 was undertaken by the writer. The results 

 of these studies, in so far as they concern the igneous geology, 

 are here briefly presented. § 



Location. 



Carrizo Mountain is located on the Navajo Indian Reserva- 

 tion in the extreme northeastern corner of Arizona. The area 

 of which it is the central feature, and which is discussed in this 

 paper, lies for the most part within Arizona (see map, fig. 1), 

 but embraces also a strip of country about three miles wide 

 across the border in New Mexico. Rising as it does 2000 to 

 3000 feet above the surrounding plain (fig. 2), " an igneous 

 island in a sedimentary sea," Carrizo Mountain forms a prom- 

 inent landmark visible for miles in every direction, except to 

 the south where the view is interrupted by the Boundary 

 Mountains. 



General Features of the Igneous Geology. 



The evidences of igneous activity are now preserved in the 

 Carrizo district in the form of various intrusive bodies, sheets, 

 sills, dikes, and the main large intrusion, a laccolith. It is 

 inferred, however, from the presence of a series of six volcanic 

 plugs just southeast of the mountain that igneous activity was 

 not confined to intrusion but manifested itself as well in extru- 



* Published by permission of the Director of the IT. S. Geol. Survey. 



f Holmes, W. H., U. S. Geol. and Geog. Survey Terr., embracing Colo- 

 rado and parts of Adjacent Territory, 1877, pp. 274-276. 



X Prof. Paper, U. S. Geol. Survey, in preparation. 



§The results of the entire investigation, embodied in a report, constitute 

 the thesis submitted as partial fulfilment of the requirements for the degree 

 of Doctor of Philosophy at Yale University. 



Am. Jour. Sci. — Fourth Series. Vol. XLII, No. 250.— October, 1916. 

 24 



