GEOLOGY OF LITTLE ROCKY MOUNTAINS 4II 
Alabama mine shows a porphyry, varying slightly from that of 
the main ridge. Its most noticeable character, however, consists 
in the prominent white crystals with which it is dotted. The 
rock shows smooth, slickensided surfaces, and in general crush- 
ing and rock movement. 
The contact between the porphyry and the Archean schists 
was observed on the high ridge east of Antoine Butte. Here 
there is an exposure of schists some 200 yardsacross, which shows 
on the crest of the ridge. The porphyry immediately overlying 
the schists is somewhat decomposed, and the contact is marked 
by five feet or so of green clays. The contact has a dip to the 
south of about 30°. 
Parting. —The parting varies in different parts of the mass. 
At the west base of Mission Butte the porphyry has a platy 
parting or lamination near the contact with the limestones, and 
the rock is dense and resists weathering so well that the contact 
is marked by a wall rising above the adjacent slope of shale and 
porphyry. At Mission and Granite buttes the rock is a granite 
porphyry, breaking into massive blocks—the normal jointing of 
a granular rock. Throughout the mountains generally, however, 
the porphyry upon weathering breaks into rather small angular 
fragments, usually but a few inches across, and forms broad 
slopes of débris covering the mountain sides. 
Vent—The rock body is in the diagram shown extending 
downward at the side of the arch, but it seems evident that the 
injection did not take place through the entire circumference of 
the ring. It becomes, therefore, a matter of interest to locate, 
if possible, the vent by which the rocks reached the horizon in 
which they were intruded. The uniform granularity of the rocks 
observed shows that there is no large stock or core which may 
be supposed to represent the source of supply. In fact all the 
indications show that the intrusion is analogous in form to a lac- 
colithic sheet. 
South of Mission Butte the slopes back of the Goldbug mine 
show outcrops of breccias formed of fragments of porphyry with 
large blocks of quartzite, the latter rock resémbling that forming 
