76 SNAKE EIVER PLAINS OF IDAHO. [bum* 199. 
on account of rotation during- their flight assumed more or less regu- 
lar spherical forms. Of these volcanic bombs, several varieties were 
recognized, the differences they present being due to variations in the 
degree of plasticity of the lava as it left the pool of molten rock from 
which it was blown and the length of its flight. 
The most characteristic of the bombs are oval and in size and shape 
frequently resemble a modern football with a projection at each end of 
its longer axis. These characteristic projections, or "ears," show a 
well-defined spiral twist, as is illustrated on PI. XII. The surfaces 
of these bombs are black and glossy when fresh, and without vesicles 
visible to the eye, but sometimes exhibit more or less spiral lines in 
relief, which are in a general way parallel with their longer axis and 
unite with or are prolonged into the twisted ridges on the projections 
at their extremities. At times also their surfaces are crossed by shal- 
low cracks, as is shown in PI. XII, which appear to have formed in a 
thin outer crust, and to have been widened by a slight subsequent 
swelling of the still viscous mass within. In size they vary from less 
than 1 inch to several feet in length, not including the projections at 
the ends, which are usually broken, but in some instances are 3 or 4 
inches long. Some examples are nearly spherical, and the "footballs" 
are sometimes slightly flattened parallel to the longer axis. The one 
shown at the bottom of PL XII is 13 inches long, including the rem- 
nants of the projections at the ends, and 7.9 by 4.7 inches, respectively, 
in its two diameters at right angles to the axis about which it was 
rotated." The largest specimen of this shape seen measured 9 feet in 
length and 12 feet in circumference. The nearly spherical bomb 
represented on PL XII measures 5.5 by 1.5 by 4.3 inches. In some 
instances the bombs, whether nearly spherical or markedly ovoid, 
appear to have been formed by the rolling together of irregular cakes 
of plastic lava, the edges of which form ridges and curved lines on 
the surface of the infolded mass, as may be seen in the photograph 
to which attention has been directed. 
Of the bombs having the external characteristics just described there 
are at least two well-marked varieties, as is discovered on breaking 
them. In certain instances there is a distinct outer crust or rind, 
composed of compact black basalt, measuring in general from a 
tenth to a fifth of an inch in thickness, and inclosing a highly vesicu- 
lar interior; in other examples there is no outer rind, and the mass is 
compact, or nearly so, throughout. 
In the case of the bombs having a rind and cellular interior there is 
sometimes a thin outer layer, perhaps a third of an inch thick, which 
is scoriaceous and merges with the still thinner compact crust inclosed 
aThis specimen is not from the Cinder Buttes, but from a much older crater, only a portion of 
which remains, near the north base of Big Butte. 
