216 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



the mountain thoroughly 1 . The coarsely crystalline bodies of rock noticed 

 by Charles Darwin in the Andes, and found by Prof. A. Stelzner in the 

 andesite lavas of Argentina, lack satisfactory description. 



The volcanic center at Electric Peak and Sepulchre Mountain, in the 

 Yellowstone National Park, described in a previous chapter, 2 furnishes 

 indisputable evidence of the relationship in question and of its former 

 existence as the center of a volcano. However, the geological structure of 

 this locality does not meet our expectations of what a great volcano should 

 look like when deeply eroded. In fact, profound faulting and extensive 

 erosion have left very little of the original volcanic pile. 



But no simpler or more obvious model of the internal arrangement of 

 a great volcano could be wished for than that which is exhibited by the 

 deeply cut valleys and steep, high ridges that constitute the drainage basin 

 of Crandall Creek and its immediate vicinity. The coarsely crystalline 

 gabbros and diorites, with smaller bodies of granite, exposed for a height of 

 3,000 feet, are plainly seen to have been intruded within a vast accumulation 

 of basaltic tuff and scoriaceous breccia, which they have metamorphosed. 

 From this coarsely crystalline mass as center, dikes of fine-grained rock 

 penetrate the surrounding lavas in all directions, the dike rocks becoming 

 finer grained rapidly as they leave the once heated core. They form a net- 

 work of branches which connect the outlying aphanitic and characteristically 

 volcanic rocks with the more crystalline dikes near the core, which finally 

 merge into the granular body of the gabbro and diorite. The whole com- 

 plex is so intimately interwoven that there is not only no possible doubt as 

 to the relative time of eruption of the glassy basaltic scoria and lavas and 

 of the gabbro and diorite, but there appears to be no ground for designating' 

 a part only of the rocks involved in the complex as volcanic. 



GEOLOGICAL DESCRIPTION. 



GENERAL FEATURES. 



The tract of country embraced in the description of the volcano of 

 Crandall Basin lies immediately east of the northeastern corner of the 

 Yellowstone Park, and includes an area somewhat larger than that drained 



1 Also, A dissected volcanic mountain; some of its revelations: Am. Jour. Sci., 3d series, Vol. 

 XXXII, No. 190, Oct., 1886. 



2 Page 89. See also The mineral composition and geological occurrence of certain igneous rocks 

 in the Yellowstone National Park : Bull. Pliilos. Soc. Washington, Vol. XI, pp. 191-220 ; and The eruptive 

 rocks of Electric Peak and Sepulchre Mountain, Yellowstone National Park : Twelfth Ann. Rept. U. S. 

 Geol. Survey, Part 1, 1891, pp. 569-664. 



