THE ORIGIN OF IGNEOUS ROCKS. 171 
Immediately over these basalts is a sanidine-mica-tuff like 
the pebble found at the locality farther north. The great 
body of volcanic rocks in which these exceptional varieties 
occur is normal andesite of various kinds, in which no crys¬ 
tals of sanidine have been developed. 
A third occurrence of alkali basalts is at Two Ocean pass, 
18 miles south of Yellowstone lake. There are five sheets 
of basalt of different habits; two are characterized by abun¬ 
dant microscopic sanidine in the groundmass, besides some 
analcite. One has abundant phenocrysts of olivine and 
augite, and the other has few phenocrysts of these minerals 
and some of oligoclase. The chemical composition of the 
orthoclase-basalt, analysis 17, shows it to be similar to the 
leucite basalts just mentioned, and also to the minettes, 
analyses 7,10,18, two of which, however, are lower in silica, 
and the other somewhat higher in alkalies. 
One of the basalt flows is rich in phenocrysts of augite and 
olivine, with none of feldspar, and if analyzed would un¬ 
doubtedly be found to be high in magnesia and lime and 
low in alumina, which would correspond to the camptonitic 
varieties of the dike rocks. 
These basalts are associated with the same kind of sani¬ 
dine-mica-tuff that occurs on Beaverdam creek. In both 
cases this tuff is so filled with fragments of basalt and ande¬ 
site that it is not possible to determine its chemical com¬ 
position. 
In • all these cases the closely associated alkali basalts 
and sanidine rocks are the extreme forms of differentiation 
of the general magma and are the surface equivalents of 
certain varieties of the olivine-augite-minettes, kersantites } 
and syenite-porphyry or trachyte. The phenocrysts are 
alike in the corresponding forms of the different varieties, 
but the groundmass has crystallized differently. 
A surface flow of sanidine lava occurs in the andesitic and 
basaltic breccias and flows immediately north of the Park. 
It is glassy and approaches rhyolite in microstructure'and 
composition, but carries no quartz phenocrysts. The por- 
