IGNEOUS ROCKS IN YELLOWSTONE PARK. 
203 
ent in the two groups, being more highly crystalline in the por- 
phyrites—many of the andesites being glassy. Many of the 
finer grained diorites have a habit, derived from the distri¬ 
bution of the ferromagnesian silicates and larger feldspars, 
which resembles that of some of the andesites and dacites 
which correspond to them chemically. But the coarse grained 
diorites bear no resemblence to the andesites in outward ap¬ 
pearance or in mineral composition. 
Chemical Characters of the Rocks. 
The chemical investigation of these rocks not only brings 
out more clearly the similarity between the two groups of 
rocks under consideration, but establishes their chemical 
identity, and permits of the comparison of those modifica¬ 
tions of the magmas which bear no mineralogical or struc¬ 
tural resemblance to one another. 
The analyses of the rocks occuring at Electric Peak, to¬ 
gether with analyses of two of the sheet-rocks, are given in 
Table VI, p. 206. They were made by Mr. J. E. Whitfield. 
With the exception of the two analyses of sheet-rocks, which 
are introduced for comparison, they have all been made from 
rocks occurring within the area of the main stock in Electric 
Peak, and represent the composition of various forms of 
the diorite and the diorite-porphyrite. The first three an¬ 
alyses are from the main body of coarse grained diorite, 
and show its variation between 56 and 58 per cent, of silica, 
with irregular variations in the other chemical constituents, 
especially in the alumina and magnesia. The next two 
analyses are from porphyrites occurring in sheets, which 
resemble certain of the porphyrites occurring in dikes and 
have been introduced in the table to show their similarity, 
and to fill up a gap between Nos. 269*2 and 3008; it not 
being advisable to multiply too largely the chemical work 
on this particular group of rocks since others of equal im¬ 
portance command their share of attention. 
That this insertion is admissible is shown by the accom¬ 
panying list of silica percentages, determined for different 
modifications and varieties of this group of rocks. 
