gfi SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
30 miles." The descent of the surface of the lava in this distance is 
about 1,000 feet, more than one-half of which is, I judge, within the 
first 10 miles. In the portion nearest the Cinder Buttes the descent 
of the surface toward the south can be detected by the eye, but beyond 
a distance of about 10 miles, the rough lava sheet seems to be level. 
The fact that the lava flowed so far on such a gentle gradient shows 
that it was highly fluid and spread almost with the freedom of water. 
The sheet produced must, in consequence, be thin, unless depressions 
were present in the surface of the plain over which it spread itself. 
All these features are such as pertain to yet other lava streams about 
the Cinder Buttes, as well as to the older lava sheets beneath the Snake 
River Plains, exposed in part in section in the walls of Snake River 
Canyon. 
In addition to the lava streams briefly described above, there is 
another immediately at the west base of the highest of the Cinder 
Buttes, which went southward, and is somewhat older than the neigh- 
boring stream on the west. Two other streams, differing in age, went 
northward from near the same portion of the belt of craters. One of 
these is comparable in extent and volume with the lava sheet which 
flowed southward. There are thus at least six recent lava flows about 
the Cinder Buttes which spread widely on the adjacent plain and in 
fact nearly surround the most prominent portion of that belt of volca- 
noes. Other lava flows from craters farther eastward were seen, but 
all of them are apparently older than those just noted, although in 
certain instances they are sufficiently fresh to be easily traced to the 
craters from which they emerged. 
The area occupied by the recent lava forming the six flows described 
above is by estimate between 250 and 300 square miles. There is no 
ready means of accurately measuring the thickness of these sheets, but 
they are certainly comparatively thin, as is apparent from the fact that 
they spread widery on a nearly level plain and cooled with essentially 
flat surfaces. Judging from the thickness of the various sheets at their 
margins and the gradients of their surfaces, their average thickness 
seems to be between 50 and 100 feet, or approximately 75 feet. On 
this basis the volume of lava they contain is between 7 and 8 billion 
cubic feet. 
In reference to certain general hypotheses concerning the origin of 
volcanoes, the static equilibrium of the earth, etc., it is of interest to 
note that the lava poured out by the volcanoes which formed the Cin- 
der Buttes was forced up to a horizon about 6,000 feet above the sea, 
"While my observations show that the northwest and southwest flows, starting from within a dis- 
tance of a few rods of each other, were contemporaneous and united with each other, the place of 
junction of the southwest flow with the next lava stream to the east has not been examined, and 
in mi what source the vast lava sheet to the south of Cinder Buttes was mainly derived is not known. 
Seemingly the southwest flow, and the next one to the east, are of the same age, and unite to form 
one sheet on the plain to the south. 
