56 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bttll.217. 
which the surface of the hill is composed. This gulf has a nearly 
vertical wall from 40 to 70 feet high, runs about east and west, is from 
500 to 800 feet across, and fully 2,000 feet long. Within it are several 
irregular ridges formed by the edges of large tilted blocks of the fallen 
crust. The lava exposed in the walls of the break is irregularly 
columnar, and the topmost layer, which arches over the summit of 
the hill, is about 40 feet thick. This sheet is continuous from one 
side of the hill to the other, passing over the summit, but in the cen- 
tral part of the dome thus formed the rock is highly scoriaceous and 
the bedding less distinct than at the sides. From the gulf in the sum- 
mit of the hill branching fractures extend down its sides to the east 
and southeast, and in part these radiating breaks are gulfs produced 
by the subsidence and tilting of large blocks of the surface layer. To 
the north of the hill just described there are other elevations of a 
similar nature, but lower and less broken, and about the group there 
are rough lava flows, which came from it and spread over the pre- 
viously eroded land. On one of these lava sheets there are small 
driblet cones, two of which are represented on PL XI, B. 
The interpretation of the facts in reference to the domes of lava 
briefly described above seems to be that from certain of the Diamond 
Craters, which were probably lapilli cones like those of that nature 
still remaining, great quantities of lava were extruded in a liquid or 
plastic condition, which buried or carried away the preceding craters 
of lapilli and, thickening about the opening from which it came, built 
up rounded hills. The outwelling lava flowed down the sides of the 
hill and a thick crust was formed on its surface. After this, the out- 
flow continued beneath the stiffened surface, and finally, when no 
more lava rose from the conduit beneath, a considerable portion of 
the surface crust fell in, leaving the black gulf now forming such a 
conspicuous feature of the summit and eastern side of the largest 
dome. The smaller lava cones or domes to the north of the principal 
cone were less fractured than the main one of the series, and retain 
nearly all of their constructional features unmodified by either frac- 
ture, subsidence, or erosion. In fact but slight changes by erosion are 
anywhere visible throughout the entire group of craters. 
The rounded hills of lava among the Diamond Craters are, as may 
be judged from the description just given, examples of lava cones, 
and are similar to many other elevations in Idaho and Oregon, which 
were produced in each case by the escape of lava in large volume 
from a volcanic conduit. The chief differences between the lava 
cones here considered and those of the normal type are the promi- 
nence of the rounded central hills and the comparatively small extent 
of the surrounding lava fields with which they merge. The usually 
characteristic profile presented by lava cones, consisting of long, gen- 
tle slopes leading up to a low central flat-topped butte, is not present. 
In this connection it may be suggested that the lava poured out to 
