42 GEOLOGY OF SW. IDAHO AND SE. OEEGON. [bull. 217. 
which still adhere to its lower surface. On account of its plastic 
consistency it did not break when it fell, but has since been fractured, 
probably on account of changes of temperature. 
Many projectiles, either complete or more or less shattered, of the 
same general character as the one shown on PI. VIII, occur about the 
Cinder Buttes, and in most if not all instances they are composed of 
dense, compact, light-colored rock, which on freshly fractured sur- 
faces looks not unlike hard-burned stoneware and resembles the 
material forming the immediate borders of basaltic dikes or, the whole 
of very thin dikes of similar character. It is evident that the lava 
forming the irregular shred-like bombs cooled quickly from a liquid 
or highly plastic condition, and that the steam and gasses contained 
in the original magma escaped, for the most part, without leaving 
vesicles. 
When the bombs are broken so as to expose their interiors they 
present at least three well-defined variations in structure. Certain 
ones, including the elongate, twisted forms described above, are 
compact throughout or exhibit only irregular and, frequently, much 
extended cavities, such as steam or gases leave in cooling lava, and 
are either light, and in fact almost white, or glossy black in color. 
In sections at right angles to the longer axis something of a spiral 
arrangement of the steam cavities, cracks, etc., is apparent, and on 
their exterior there is more or less evidence of the influence of rota- 
tion while the mass was still plastic. The bombs having these char- 
acteristics seem to have been formed of highly plastic or nearly liquid 
lava, from which the contained steam and gases escaped freely. 
Bombs of a second variety, characterized by their rudely spherical 
shapes, rough exteriors, and highly vesicular and frequently nearly 
hollow interiors, but without surface crusts, spiral lines or ridges, or 
projecting ears, occur in abundance, especially in the walls of para- 
sitic cones or what may perhaps be termed driblet cones of large size. 
These rough spheres range in diameter from a few inches up to 2 feet 
or more, and the walls of the large cavities within frequently present 
the appearance of "pulled dough," as if marked expansion of the 
steam cavities had occurred while the material was viscous. (PL 
VII.) The bombs of this variety are sometimes flattened and bear on 
their under surfaces impressions of the material on which they fell. 
At times they adhere one to another, and in some instances were suffi- 
ciently plastic to flow after coming to rest. The occurrence of these 
bombs in chimney-like piles, about openings in lava streams, and in the 
walls of the so-called ovens to be described later, indicates that they 
were tossed out of openings in the crusts of lava streams, but did not 
make any considerable journey in the air. They form a connection 
between projected clots of vesicular lava, such as are commonly 
termed "cinders," and true bombs, which were thrown to a consider- 
able height and cooled while rotating. 
