410 WEED AND PIRSSON 
east, as actually observed north of Shellrock Butte, with a 
thickening of the mass beyond. While the observed facts seem 
to indicate a fairly uniform contact plane between the schists 
and the base of the porphyry, the upper surface is not so regu- 
lar, as is shown by the occasional occurrence of areas of lime- 
stones as blocks of warped strata at elevations considerably 
below the highest porphyry peaks. The porphyry intrusions of 
the marginal summits, such as Crown Butte, Indian Butte, and 
similar elevations, appear to be small laccolithic bodies of the 
porphyry breaking through the massive limestones, as is the 
case at Indian Butte, where the igneous rock occurs in contact 
with the black shale of the Cretaceous. 
If the soft nature of the shales was the determining factor 
for the intrusion of the porphyry at this horizon, then the part- 
“ing of the beds by the intrusion would be between quartzite and 
shale; this is, however, not always the case, for the quartzite at 
- the base of the Palzozoic series is sometimes found beneath the 
porphyry, between this rock and the crystalline schists, and 
sometimes above it, and beneath the soft Cambrian shales. 
No dikes or ordinary intrusive sheets were observed in the 
mountains, nor are there any extrusive rocks either here or in 
the immediate vicinity. 
The gulches which cut the porphyry areas are very deep, 
with steep slopes heavily timbered with pole pine from three to 
twenty feet high, and the creek bottoms can only be traversed 
with difficulty. The porphyry is some three or four hundred 
feet thick upon the principal summits and rests upon black mica 
schists, into which the streams have cut their gorges, the walls 
on either side showing typical Archean exposures capped by great 
débris slopes of porphyry. The character of the rock formation 
is readily indicated by the vegetation, as the porphyry exposures 
and débris slopes are everywhere covered by young pines or, 
more rarely, form smooth grassy slopes, while the limestone and 
schist areas are covered by the big-leafed pine, which forms open 
groves and show occasional rough outcrops of the country rock. 
The ridge extending southward from the main divide to the 
