IGNEOUS ROCKS IN YELLOWSTONE PARK. 
213 
blende and biotite and no pyroxene, the relative proportion 
of these minerals varying. The character and amount of 
the feldspars differ in these rocks, and so does the abund¬ 
ance and mode of occurrence of the quartz. In Nos. 3017 
and 3680 quartz appears in porphyritical crystals; in the 
other rocks it is confined to the groundmass. 
Comparing the two sets of analyses and the correspond¬ 
ing diagrams it is evident # that the intrusive rocks of 
Electric Peak and the volcanic rocks of Sepulchre Moun¬ 
tain have the same chemical character. In fact, they 
are chemically identical, for the varieties that have been 
analyzed are but a few of the many mineralogical and struc¬ 
tural modifications assumed by these series of rocks. The 
analyses serve as indications of the range of the chemical 
variability of the magma or magmas furnishing these rocks. 
Furthermore the comparison demonstrates that the mag¬ 
mas that reached the surface of the earth in this place had 
exactly the same chemical composition as those which re¬ 
mained enclosed within the sedimentary strata. It proves 
with equal clearness that the different conditions attending 
the final consolidation of the extravasated and of the intruded 
magmas affected not only their crystalline structure but their 
essential mineral composition . The most marked illustration 
of this is in the occurrence of biotite in the two series. In the 
* 
volcanic rocks of this locality biotite is an essential constit¬ 
uent of the more siliceous varieties, and is only rarely found 
as an accessory constituent of the varieties with less than 61 
per cent, of silica. In the intrusive rocks it is an essential 
constituent of all the coarse grained varieties, even the most 
basic. In the finer grained porphyritic forms it is a constit¬ 
uent of the groundmass to a variable extent. The second 
most noticeable difference is the presence of considerable 
quartz in the coarse grained forms of the basic magmas, and 
its absence from the volcanic forms of the same magmas. 
Correlation of the Rocks on a Chemical Basis. 
Correlating the two groups of rocks according to their 
chemical composition as in Table XI, we see that the horn- 
