38 GEOLOGY OF SW. IDAHO AND SE. OREGON. [bull.217. 
orifice from which it came and flowed outward in all directions, form- 
ing what it is convenient to term lava cones. The cinder cones are 
high in reference to their diameters, with steep and normally concave 
slopes, while the lava cones are usually low, with gentle and at times 
convex sides and immensely expanded bases. 
One of the most notable facts connected with the volcanoes in ques- 
tion is the vastness of the lava fields spread out about them in com- 
parison with the size of the cinder or lava cones found at the summits 
of the conduits from which the material came. 
On the surfaces of the lava flows there are two principal features, 
which are dependent on the ratio between the motion of the lava and 
the degree of rigidity of the crust formed on its surface as cooling 
progressed. When the cooling and stiffening of the surface occurred 
without marked disturbance from the flow of the still liquid material 
beneath, smooth, swelling, convex surfaces resulted, which at times 
were wrinkled and even forced upward into hollow folds. These folds 
show the characteristics of pahoehoe, as such surfaces are termed in 
the Hawaiian Islands. (PI. XII, X) When, however, the underflow 
beneath a brittle crust caused it to become fractured, and the frag- 
ments thus produced were tossed about in much the same manner 
that the cakes of ice in an ice jam are crushed together and piled up, 
a rugged surface, simulating what is termed aa in the Hawaiian 
Islands, resulted. (PI. XII, B.) 
The numerous instructive features just referred to, which were 
described and discussed in some detail in the report on the observa- 
tions made in 1901, a were found in the crater and lava flows of south- 
eastern Oregon examined in 1902, and still other phenomena belong- 
ing to the same general category were observed. The reexamination 
of the Cinder Buttes enabled me to verify and extend the results 
previously recorded. 
CINDER BUTTES. 
The Cinder Buttes are situated on the west side of the Snake River 
Plains, about 70 miles west of Blackfoot, Idaho. They are not con- 
spicuous on account of size, as the highest crater in the group rises 
but 600 feet above the adjacent plain, but are remarkable for the 
vast amount of lava that flowed from them and for the many instruc- 
tive details they present in reference to the behavior of volcanoes 
which erupt basic and easily fusible lava. 
The additional facts concerning this most interesting group of cra- 
ters and lava flows, at least in a measure supplementing the account 
of them already published,* relate principally to the floating away of 
large fragments of a ruptured tuff cone on the surface of the outflow- 
ing lava stream, a greater variety in the volcanic bombs occurring 
about the crater than was previously known, and the occurrence of 
a Bull. U. S. Geol. Survey No. 199. 
