58 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull. 217. 
the northeast. It is thus evident that the surrounding lava flow came 
entirely from this single and relatively small center of eruption. The 
lava flow referred to surrounds the crater on all sides, but extends 
farthest to the northwest, or in the direction of drainage of the region 
before the lava was spread out. The area of the lava field is by esti- 
mate fully 100 square miles. The thickness, on the supposition that 
it was spread out on a nearly level plain — which was apparently the 
case — varies from but a few feet at the margin to about 400 feet at 
the center. The surface of the lava is rough, and many pressure 
ridges are present, particularly within a radius of 2 to 3 miles of the 
crater. Considerable weathering has occurred, and in general the 
entire lava flow, as well as the sides and bottom of the crater, is over- 
grown with sagebrush, bunch grass, and other vegetation. On its 
southern border the lava field dammed Rattlesnake Creek, and caused 
a lake of considerable size to form. The bed of this lake has been filled 
to a depth of at least 40 feet with fine alluvium, and, owing in part 
also to the lowering of its outlet by erosion, is now dry and occupied 
by natural meadows of rye grass. 
Bowden Crater, as may be judged from the facts just mentioned, is 
a typical and most instructive example of a lava cone of the variety 
having a crater at the top. This crater is circular, and not an irreg- 
ular gulf, produced by the breaking and partial subsidence of a dome 
of lava, as in the case of the highest of the Diamond Craters. The 
precise manner in which the crater's rim was built up is not clear, but 
apparently it was formed by the radial overflow of lava in thin sheets. 
The central part is perhaps due to a drawing off of the molten rock 
which once occupied it, through tunnels in the surrounding lava. 
It is possible, in this and other similar instances, that a circular cin- 
der rim Avas formed about the summit of the conduit from which the 
lava rose by the cooling and running together of splashes of liquid 
lava. Craters formed in this manner would furnish a connection 
between those composed of lapilli and scoria and true lava domes, 
like the highest of the Diamond Craters. 
SUMMARY. 
The various phases of volcanic eruptions illustrated by the Cinder 
Buttes and by the Jordan, Diamond, and Bowden craters are such as 
pertain in general to volcanoes which, for the most part, discharge 
highly liquid lava. At each of these volcanic centers it seems that 
the first eruptions were of the explosive type, and that the elevations 
produced by the accumulation of projectiles — whether solid, plastic, 
or liquid — first formed were, to a considerable extent, and in some 
instances completely, buried by the subsequent quiet effusion of vast 
quantities of liquid lava. Among the more interesting of the minor 
phenomena associated with them are the craters which were built of 
ejected angular fragments which were blown out from the cooled and 
rigid lava at the summits of the volcanic conduits, and similar struc- 
