IGNEOUS ROCKS IN YELLOWSTONE PARK. 201 
most acid variety appears to have closed the series of erup¬ 
tions that built up Sepulchre Mountain. 
It is evident from a study of these dikes in the field that 
they were formed by a succession of eruptive actions, which 
fissured the breccias and filled the cracks with magmas that 
differed chemically and mineralogically; and that, starting 
with magmas resembling those of the latest breccias, they 
became more and more siliceous. 
The andesitic breccias and dikes form a continuous pet- 
rographical series, which may be divided for convenience 
into varieties that differ in the nature of the porphyritical 
minerals as represented in the accompanying table, plagio- 
clase being present in all. 
Table V. 
Variation of phenocrysts in the rocks of Sepulchre Mountain. 
(a) 
PYROXENE 
(*) 
pyroxene 
hornblende 
to 
HORNBLENDE 
to) 
PIORNBLENDE 
biotite 
w 
hornblende 
biotite 
quartz 
(/) 
BIOTITE 
QUARTZ 
The dike rocks are porous and vesicular in the smaller 
dikes and are sometimes glassy. In the larger bodies they 
are more compact and usually quite crystalline. 
The course of events at Sepulchre Mountain may be ex¬ 
pressed as follows, and as tabulated on the second half of 
Table IV, page 199: An acidic, andesitic breccia carrying 
a great amount of Archaean fragments was spread upon the 
surface of the country. It rests upon Cretaceous strata, and 
was probably thrown from some neighboring area of Ar¬ 
chaean schists and gneisses. Such an area at present exists 
to the north, across the Yellowstone river. 
Upon this were thrown breccias with massive flows of 
