608 THE JOURNAL OF GEOLOGY. 



head of Sunlight Basin, about fifteen miles south of the Crandall 

 center. 



The center toward which the Crandall dikes converge is a 

 large body of granular gabbro, grading into diorite. It is about 

 a mile wide, and consists of numerous intrusions penetrating one 

 another and extending out into the surrounding breccia, which is 

 highly indurated and metamorphosed in the immediate vicinity 

 of the core. Within the area of indurated breccia the dike rocks 

 become coarse grained rapidly as they approach the gabbro core. 

 This was undoubtedly the central conduit of an ancient volcano, 

 the upper portion of which has been eroded away. 



Upon comparing the geological structure of this region with 

 that of an active volcano, like Etna, it is apparent that the lava 

 flows which form the summits of the outlying peaks must have 

 been derived from lateral cones fed by dikes radiating from the 

 central conduit. And assuming that the volcano of Crandall 

 Basin was similar in type to that of Etna, an idea of its original 

 proportions is derived by constructing upon profile sections 

 through the Crandall cone the outline of Etna. If the erosion of 

 the summits of the highest peaks is neglected, the resulting 

 height of the ancient volcano above the limestone floor is esti- 

 mated at about thirteen thousand four hundred feet. This is 

 undoubtedly too low, and is well within the limits of present 

 active volcanoes. Erosion has removed at least ten thousand 

 feet from the summit of the mountain to the top of the high 

 central ridge in which the granular core is situated, and has cut 

 four thousand feet deeper into the valleys on either side. It has 

 prepared for study a dissected volcano, which, it is hoped, will in 

 time reveal some of the obscurer relationships existing between 

 various phases ot igneous rocks. 



Petrologlcal Features. — It will not be possible in an abstract to 

 do more than present, in the briefest manner, the more salient 

 features of the petrology of the rocks of this volcano. The 

 rocks are mostly the same as those in various parts of the 

 Yellowstone National Park, some of which have been described 

 in another place. The older acidic breccia consists of fragments 



