1X4 SNAKE KIVER PLAINS OF IDAHO. [bull. 199. 
up of the still plastic lava. Another step in this process is illustrated by 
spindle-shaped bodies 6 to 8 inches in diameter and a foot or more 
long, which in some examples have a scoriaceous core, involved in 
several layers of stony or glassy lava (PL XIX, B). The concentric 
white lines in this example are due to involved grains of sand. A more 
pronounced occurrence of the same nature is shown by oval or nearly 
spherical bowlder-like masses from a few inches to 2 or more feet in 
diameter, which are excessively hard and compact, The inner por- 
tions of these spherical masses are composed of stony basalt, usually 
containing kernels of olivine, which are covered with a coating vary- 
ing from half an inch to an inch or two in thickness, of black, 
-glassy lava. The outer crust in some places has a smooth, black, glit- 
tering surface, like bottle glass which while fused was poured over 
an irregular surface and allowed to cool quickly. In other places 
the surface coating of glass is rough and of a dull-brownish color. 
These balls, together with the less completely rolled-up pillow-like 
masses, are embedded in a coarse breccia consisting mainly of irregu- 
lar glassy kernels, but frequently containing also angular fragments 
of compact granular basalt (PI. XX). The spherical masses of basalt 
just described appear to be analogous to certain "lava balls" observed 
by Dana a in lava streams of the Hawaiian Islands^ These lava balls 
are stated to be commonly from 3 to 5 feet in diameter, but one con- 
tains at least a thousand tons of rock. Even the normal Hawaiian 
examples, however, are larger than the largest specimen seen by me 
in the w T alls of Snake River Canyon. 
While the upper and central portions of the lava sheets referred to 
are composed of compact, granular, and frequently columnar basalt, 
usually more or less vesicular, their basal portions have an open text- 
ure, and consist mainly of glassy material. This contrast shows that 
the upper portion cooled slowly so as to form what may be termed | 
normal basalt, while the basal portion cooled quickly. The glassy 
portion is not scoriaceous and has the appearance of obsidian, especi- 
ally when examined under a microscope with reflected light. While 
this glass is characteristically cellular, the openings are not like those 
produced in scoriaceous lava by the expansion of steam or gases con- 
tained in the magma while still hot and plastic, but they are irregular 
cavities due to a force acting from without. The walls of the cavities 
and the rough, glassy offshoots and the extremely irregular branches 
about them are not vesicular, but are composed of black glass. 
Another instructive fact is that the cellular, excessively irregular 
masses of glass, although black within, are nearly white at the sur- 
face. This is due to a thin incrustation which can not be washed off 
or removed with strong acids. Under a microscope, using reflected 
light, the incrustation is seen to be composed of white sand grains, 
a J. I). Dana, Characteristics of Volcanoes, New York, 1890, pp. 10-11, 241. 
