108 SNAKE RIVEE PLAINS OF IDAHO. [bull. 199. 
soil covered over hundreds of square miles. The occasional dome and 
pressure ridges on these older lava flows are black, the edges of the 
fissures in them yet sharp, and the corrugations on their surfaces still 
clearly distinguishable. It is evident, therefore, that the soil conceal- 
ing all but their more prominent rugosities is not derived from the 
disintegration and decay of the underlying lava. The nature, mode of 
origin, etc., of this widely extended surface sheet will be discussed 
later. 
The phenomena connected with Cinder Buttes — which are compara- 
tively small volcanic cones, or fragments of cones, built of scoriae, ] 
lapilli, bombs, etc., by explosive eruptions, and which later sent out 
truly immense volumes of highly liquid lava, which spread widely 
over the adjacent plain, and added another sheet of basalt to those 
previously formed, etc. — are all of recent date, and assist in interpret- 
ing the records of the still greater and much older eruptions, which 
formed by far the larger part of the Snake River Plains. Additional 
assistance in this same connection is furnished by other but less 
remarkable volcanoes, also of recent date, which rise above the surface 
of the same vast lava plains. 
Market Lake craters. — About 6 miles northeast of the town known 
as Market Lake there are two conspicuous buttes, which rise from 
500 to 600 feet above the surrounding plain. The smaller of the two, 
situated to the north of its companion, shown in PL XVI, A, has an oval 
base measuring, approximately, 1 b} T 2 miles, in its principal diame- 
ters, and at the top is about 3,000 feet across from north to south and 
2,000 feet in a transverse direction. It has a well-defined crater in its 
summit, still about 150 feet deep, although containing much material 
blown or washed from the encircling wall. A secondary crater is 
stated by Bradley a to occur on the northwest portion of the main 
crater, which has a diameter of 200 or 300 feet. The cone is grass 
covered and bears a scattered growth of small juniper trees. It is 
composed almost entirely of yellowish lapilli, now loosely consolidated 
into tuff, which is well stratified, as is shown in PI. XVI, B, and has 
both the inward and outward dips so common in tuff cones. The beds 
forming the outer slopes are well exposed on account of wind erosion 
and dip away in all directions from the crater's rim, at angles in gen- 
eral of about 20°, but flatten near the base, where they appear to 
become nearly horizontal. On the inner side of the craters the beds 
of lapilli are inclined downward toward its center at angles of from 
25° to 30°. b Contained in the tuff are a few angular masses of scori- 
«F. H. Bradley: Sixth Ann. Rept. U. S. Geol. and Geog. Surv. Terr., 1873, p. 210 
b Bradley suggests that the material composing these volcanic cones was deposited in water, and 
that a lake formerly existed on the Snake River Plains. My own observations failed to confirm 
either of these suggestions. The stratification of the tuff forming the craters is of the same character 
as that which is known to occur in many other cinder and tuff cones not formed in water, and calls for 
no special explanation. No evidence of the presence of a lake on the surrounding plains since the 
craters were built has been obtained. 
