bussell] OPEN FISSURES AND FAULTS. 119 
one locality to the north of the highest of the buttes a block of recent 
lava measuring several hundred feet in diameter has been elevated and 
sharply tilted, thus indicating that movements in the older rocks 
beneath the recent surface sheets have taken place. 
In the summit of Black Crater there is a gulf, not of the nature of 
a crater, but of an open cleft, in the walls of which the edges of thin 
layers of lava are broken across and well exposed. This gulf is by 
estimate 300 feet across and 100 feet deep, and traverses the summit 
of the butte in a northeast-southwest direction. In its bottom there 
is black, scoriaceous lava which rose after the fissure was opened, and 
now forms a pahoehoe floor. This large fissure joins another, or, per- 
haps more accurately, two others at its southwest extremity. These 
branching fissures, 20 to 30 feet across, traverse the flanks of the butte 
irregularly and in their walls expose admirable sections of the thin- 
bedded, highly scoriaceous lava of which the crater is composed. In 
this case it is evident that the crater has been badly shattered, not by 
a steam explosion, for there are no blocks of lava or fragments of the 
nature of lapilli scoria, etc., about the openings, but owing to an over- 
flow of still liquid lava from beneath, or by an earthquake. 
Similar open fissures occur also in certain instances in the older 
sheets of Snake River lava, and are evidently of recent origin. 
At Cleft the surface of the surrounding nearly featureless plain is 
formed by a sheet of massive, vesicular lava having a rudely columnar 
structure, which is broken by a fissure having several branches that 
run nearly east and west and can be traced for fully a mile. The 
fissure is from 5 to 10 feet wide, and is loosely filled with large 
blocks of basalt which have fallen from its side. The blocks rest 
against the sides of the cleft and support one another, leaving large 
open spaces into which a person may easily descend at least 10 feet, 
and it is evident that the opening is much deeper. The supported 
lava blocks in places form a series of floors, beneath which a person 
can clamber for distances of several hundred feet, as in a long, narrow 
cave; light from above being excluded by the roof formed of dis- 
lodged blocks. Other similar open fissures of still greater extent and 
depth are reported as occurring about 2 miles west of Cleft. In each 
of these instances there has been no displacement of the rocks forming 
the walls of the fissures, and to a person standing even a few yards to 
one side of a break no suggestion of its presence is visible on the sur- 
face of the plain. These fissures are of the same character as those 
sometimes formed during earthquakes, but instead of being due to 
earthquake shocks, they were produced, I am inclined to believe, by 
other causes, and at the time of their formation caused shocks or 
vibration in the rocks beneath the adjacent region. 
About 1 mile east of Cleft the principal open fissure changes to a 
fault, the heaved side of which stands on a vertical escarpment of 
