IGNEOUS ROCKS IN YELLOWSTONE PARK. 
215 
are the accessory minerals. The quartz contains fluid in¬ 
clusions, which complete the correspondence of this diorite 
with typical diorites of other regions. 
Conclusions. 
When we consider the geological structure of the region ; 
the occurrence at Electric Peak of a broad body of eruptive 
rocks that have broken up successively through the same 
general channel, and that have imparted sufficient heat to 
the surrounding Cretaceous strata to highly metamorphose 
them for a considerable distance; the existence of a system 
of fissures filled with dikes of porphyrite that radiate out¬ 
ward toward the south and southwest; and when we observe 
at Sepulchre Mountain that an accumulation of breccia, 
resting on the Cretaceous, is traversed by a system of dikes 
radiating outward toward the north and northeast; the 
two systems being separated and probably disconnected by 
a profound fault— 
When we further consider the petrographical resemblance 
between the dike rocks of the two places, the correspondence 
of habit between the more acid members of both series, and 
the chemical identity of the magmas involved, we feel 
justified in concluding that: 
I. The volcanic rocks of Sepulchre Mountain, and the in¬ 
trusive rocks of Electric Peak were originally con¬ 
tinuous geological bodies. 
II. The former were forced through the conduit at Electric 
Peak during a series of more or less interrupted 
eruptions. 
III. The great amount of heat imparted to the surrounding 
rocks was due to the frequent passage of molten lava 
through this conduit. 
IV. Portions of the different magmas erupted found their 
way into vertical fissures and took the form of 
dikes; portions reached the surface and became lava- 
flows and breccias, while portions remained in the 
conduit. 
26-Bull. Phil. Soc., Wash., Vol. 11 
