Russell.] RECENT VOLCANOES. 55 
connection is that the basins above some of the lava dams are still 
unfilled and have never been occupied by water bodies except ephem- 
eral or play a lakes. This evidence seems to show that the climate of 
the region has been as arid as at present ever since the lava streams 
obstructed the drainage. 
The Diamond Craters are of two types, namely, lapilli cones and 
lava cones, the lapilli cones situated in the southwest and the lava 
cones in the northeast portion of the group. The lapilli craters are 
mostly low, and range in size from one measuring about 2,500 feet 
across to small conical piles of brownish debris. Within the largest 
crater there are hills and mounds of lapilli of the same character as the 
material forming its encircling rim, but rising to a height of from 50 
to 75 feet above it. This great accumulation of fragmental material 
presents an uneven surface consisting of hillocks and crater-like hol- 
lows and bears evidence of the occurrence of weak explosions in a 
crater so abundantly charged with debris that it could not clear itself. 
From analogy with streams, the volcano may be said to have been 
overloaded with debris. The fragmental material was blown up into 
hills, and crater-like pits opened in it, but the escaping lava did not 
have sufficient force to eject it from the crater. 
The hills and rings of lapilli among the Diamond Craters present 
many variations. Some are simple conical piles of the normal type, 
with depressions in their summits; in others lava rose and, breaching 
the inclosing wall, overflowed. In one instance the lava, after rising 
in a crater and outflowing, was drawn off beneath the crust formed 
on its surface and within the bowl of lapilli, about 600 feet across, 
causing the crust to fall in and leaving a black, irregular gulf 30 
feet deep, and one lapilli ring or crater has another, composed of the 
same kind of material, within it, thus recording two stages of activity. 
In one instance a small crater is composed of compact, spherical lava 
balls or bombs, ranging in size from about 2 inches to the size of small 
shot. These bombs are rough, of a dull-red color, not cellular, and 
exhibit no evidence of rotation excepting their well-rounded shapes. 
Bordering the lapilli craters on all sides are the broad, rough surfaces 
of recent lava fields, which in most instances are covered to some 
extent with sagebrush and other vegetation. 
To the north of the lapilli craters and merging with them as topo- 
graphic forms are rounded hills composed of lava sheets, which fur- 
nish examples of lava cones. The highest of these hills rises about 
400 feet above the adjacent plains and is the highest and most con- 
spicuous summit in the group of which it is a member. This central 
dome is about 1|- miles in diameter, and so far as is indicated by the 
exposures is composed throughout of lava sheets, which occupy its 
summit and descend its sides in all directions to the surrounding 
plain. There is no true crater to be seen, but at the summit of the 
hill there is a gulf, due to the falling in of a large block of lava of 
