eussell] IRREGULAR THICKNESS OF LAVA SHEETS. 117 
glassy basalt, while the upper part is granular and irregularly colum- 
nar. Involved in the glassy material forming the base of the sheet 
of basalt are a few small masses of soft, white, sandy clay, like the 
bed beneath. This indicates that the basalt flowed into a lake, and at 
its base was quickly cooled to a glass, which was expanded and made 
cellular by the steam generated. The striking feature of the exposure 
is that the soft white lake beds on which the lava rests are not hardened 
and are undisturbed, the lamina being still horizontal, even to the con- 
tact with the lava. Some of these features may be recognized in the 
photograph reproduced on PI. XXI. The only change is a slight 
yellow stain, perceptible to a depth of 5 or 6 inches from the surface 
of the contact downward. This discoloration may have been produced 
subsequent to the coming of the lava and due to weathering. 
The characteristics just described occur not at one small exposure 
only, but at several throughout a distance of about 600 or 800 feet. 
How a sheet of lava 30 to 40 feet thick could advance over a bed 
of soft unconsolidated sandy clay, which was evidently covered with 
water, without disturbing it in the least degree, is difficult to explain. 
Possibly the lava sheet may have advanced by sending out small thin 
streams of liquid rock of too little weight to disturb the beds on which 
they rested, which were later overridden by the main flow, but that 
such was the case is not apparent in the internal structure of the lava 
itself. 
IRREGULARITIES IN THE THICKNESSES OP LAVA SHEETS. 
An interesting fact in connection with the lava sheets exposed in 
section in the walls of Snake River Canyon below Shoshone Falls is 
that in several instances they exhibit conspicuous variations in thick- 
ness while their surfaces remain essentially horizontal. The obvious 
explanation of this is that the surfaces over which the lava flowed were 
irregular. 
Examples of the phenomena referred to occur in the north wall of 
Snake River Canyon in the southeast portion of Ada County, where, 
as already stated, the rim rock forming the brink of the canyon, and 
usually having a thickness of 30 to 40 feet, thins to a few feet, and 
for a space a mile or more long is wanting. The beds of lapilli which 
occur normally below the sheet of basalt rise and protrude through it 
so as to form low mounds which rise higher than the adjacent surface 
of the lava. In this instance the lava flowed against a hill of lapilli, 
probably completely surrounding it, and became thin and finally failed 
to cover the lower terrane on account of an elevation on the general 
surface over which it advanced. 
Again, in the neighborhood of the mouth of Rattlesnake Creek 
there is a sheet of lava 40 feet thick which for a distance of 2 or more 
miles forms a conspicuous terrace on the north side of the canyon. 
