IGNEOUS ROOKS IN YELLOWSTONE PARK. 
195 
other than plagioclase, which is common to all the varieties, 
as in the following table : 
Table I. 
Variation of Phenocrysts in the sheet-rocks at Electric Peak. 
pyroxene 
hornblende 
hornblende biotite 
HORNBLENDE BIOTITE 
hornblende biotite quartz 
The variations in the mineral composition are accom¬ 
panied by changes in the character and amount of the feld¬ 
spars. Toward the end of the series in the order given the 
feldspars become less basic and more abundant and are as¬ 
sociated with an increasing amount of quartz in the ground- 
mass of the rock. The proportion of ferromagnesian silicates 
decreases from the pyroxene end toward the mica end of the 
series. 
The intrusion of these sheet-rocks was not the result of a 
, single eruptive action, but of a succession of eruptions; 
hence we observe that there was a period of volcanic activity 
that injected into the mass of Electric Peak a series of mag¬ 
mas which solidified as pyroxene-porphyrite, hornblende- 
porphyrite, and hornblende-biotite-porphyrite. 
Subsequently another period of activity filled the eastern 
portion of the mountain with dikes and the stock. The 
magmas forced up at this period solidified as porphyrites 
and diorites, which grade into one another structurally and 
mineralogically. The coarse grained rocks, the diorites, 
occur in the stock and larger apophyses; the porphyrites 
occupy the dikes and smaller apophyses, and are found 
along the sides of the stock in places, in contact with the 
sedimentary rocks. 
The great body of diorite varies both in structure and 
composition; the variations being rapid in some places, and 
very irregular. It is mostly a dark-colored granular rock, 
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(e) 
