GEOLOGY OF LITTLE ROCKY MOUNTAINS 417 
present. The rock is somewhat decomposed, and the cavities 
are filled with rusty material which produces the yellowish tint 
of the rock. 
The micro-section shows large crystals of feldspar as pheno- 
crysts in a fine-grained groundmass of feldspar, with some quartz. 
The rock is dotted with fine particles of iron ore and probably a 
very little biotite was formerly present. The description given 
of the rock from the main crest, p. 413, will apply perfectly to 
this rock except that the orthoclase phenocrysts are at times 
quite large, and on the whole there is less quartz in the fine, 
microcrystalline, granular groundmass. The feldspar phenocrysts 
are very fresh, clear, and unaltered, the groundmass rather turbid. 
On the north slopes of Mission Butte the underlying schists 
are exposed beneath the porphyry, the rocks being dark and 
micaceous. The hilly country lying between Mission Butte and 
the limestone ridge which forms the northern limit of the moun- 
tain mass is devoid of large timber, and the surface appears to 
be covered entirely by porphyry. A specimen obtained on 
Peoples Creek, a short distance above the saw-mill, shows a 
fine-grained rock, breaking readily into large débris blocks 
which cover the mountain slopes. The rock is of a buff color, 
but the dark-colored outcrop is so covered by lichens as to 
closely resemble the quartzites of the Cambrian and deceive the 
observer. The rock is found just above the Indian’s saw-mill, 
where it occurs near the contact with the sedimentary rocks. It 
represents a variety not noticed elsewhere in the mountains. It 
is a compact, dense rock of a decided pinkish buff color, with 
abundant small phenocrysts of feldspar which are sometimes 
tabular, and with occasional grains of quartz 1™™ across. The 
rock shows small cavities due to the decomposition of some 
ferro-magnesian mineral, which on the weathered surface form 
small pits that are quite conspicuous. 
This type is quite similar to that forming Antoine Butte and 
the central mass of the mountains, except that the phenocrysts 
are smaller and more thickly crowded. There is also less oligo- 
clase and nearly all of the phenocrysts are of the type of the 
