198 
IDDINGS. 
change in the character of the groundmass, which is richer 
in quartz and assumes a more granular structure toward 
the mica end of the series. 
The course of volcanic events at Electric Peak, then, com¬ 
menced with successive intrusions between the sedimentary 
strata of a series of porphyrites. A later series of magmas 
came up through vertical fissures and a central conduit. 
These later magmas imparted a great amount of heat to the 
rocks immediately surrounding the conduit, and highly 
metamorphosed them. The magmas which followed one 
another through the conduit broke through those that had 
previously solidified within it, but had not entirely cooled. 
They solidified in the outlying, narrow fissures as dikes of 
porphyrite, while within the heated conduit they consolidated 
into coarse grained diorites of various kinds ; the magmas 
of this series of eruptions becoming more and more siliceous. 
The succession of eruptions may be expressed as in the 
first half of the table on the opposite page. 
Sepulchre Mountain .—The eruptive rocks lying east of the 
fault line embrace a large body of extravasated breccias 
with smaller bodies of dikes that have broken through the 
breccia. The whole group of rocks coming within the 
generally accepted class of volcanic rocks. 
The breccias which form the main mass of Sepulchre 
Mountain and extend as far west as the fault consist of frag¬ 
ments of various kinds of andesites. The lowest portion is 
made up of acidic andesites, that are characterized by pheno- 
crysts of biotite, hornblende and plagioclase. They are 
light-colored rocks, passing in places into fine tuffs. This, 
the oldest breccia, carries a great amount of Archaean ma¬ 
terial in fragments, which are not found in the later, over- 
lying breccias. Over the acidic breccia is a dark-colored 
one of basic andesite with inconspicuous phenocrysts of 
pyroxene and plagioclase, and a variable amount of horn¬ 
blende. Higher up the breccia becomes grayer, and carries 
more hornblende phenocrysts. Some of the dark-colored 
