24G GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



variable characters which express the active principles of vulcanism whose 

 laws are the ultimate object of our investigations. Hence it is that in the 

 treatment of intricate groups of rocks belonging to any one region we are 

 constantly confronted by questions as to the best methods of studying and 

 of describing the variability of the rocks. In the present instance it will 

 be most advantageous to proceed with a consideration of the development 

 of crystallization in the core of the volcano, postponing the description of 

 the remaining dikes in order to connect them more closely with other 

 bodies of similar rocks from different parts of the Yellowstone Park, which 

 will be described collectively in another chapter (Chapter IX). 



GRANULAR CORE AND INTERSECTING DIKES. 



Gabbro and gabbro-porphyry. — The western and central portions of the granular 

 core have been explored with as much care as the time allowed, but the 

 eastern part below the summit of the plateau was not visited. The main 

 mass consists of coarsely granular rock intersected by dikes or veins from 

 20 feet to 10 inches in width, which are more noticeable in the margin of 

 the core, where they exhibit a radial arrangement. The character and 

 composition of the g'ranular rock vary to some extent. It is a gabbro — 

 that is, it consists primarily of labradorite-bytownite and pyroxene, with 

 biotite and very little, if any, hornblende, with some orthoclase and a little 

 quartz, and grades into facies which are quartz-diorite. In places the 

 alkalies are higher than in normal gabbro or diorite and lead to the produc- 

 tion of considerable orthoclase, yielding varieties of rock approaching- 

 monzonite, and in some cases being monzonite. 1 It becomes finer grained 

 toward the margin of the core, and an idea of its microscopical character- 

 istics is best obtained by following the changes from fine to coarse grain 

 through two series of specimens collected for the purpose. 



One series of nine specimens represents the modifications which have 

 taken place within a distance of 100 feet. The finest-grained form (1388) 

 is near the southwest margin of the core on the southwest spur, at a spot 

 225 feet higher up the slope than the specimen (1383) already described, 

 which was collected from the massive exposures and which has been called 

 a basalt-porphyry. The two rocks resemble each other in the character of 

 their phenocrysts, but the latter has a finer-grained groundmass. 



1 Brogger, W. C, Die Eruptivgesteine des Kristianiagebiet.es : II. Die Ernptionsfolge der tria- 

 dischen Eruptivgesteine bei Predazzo in Siidtyrol, Christiauia, 1895. 



