102 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
thus appears that there are depressions in the surfaces of the older 
lava sheets which are analogous to those in the younger lavas, and are 
probably also of similar origin. 
MARGINS OF LAVA FLOWS TERMINATING ON A PLAIN. 
The principal variations in the character of the margins of lava 
sheets which expand on a plain are determined by the plasticity or 
rigidity of the lava. If the lava is highly plastic, or liquid, it termi- 
nates in a low frontal slope, having the characteristics of pahoehoe; 
in such instances it may thin out to a feather edge, only a few inches 
thick, with local extensions here and there, which are highly scoria- 
ceous within and have oval and frequently wrinkled surfaces. When 
lava becomes thick and viscous, and especially if blocks formed by the 
breaking of a rigid crust are inclosed by it, its flow is retarded and its 
margin presents a steep slope. In such instances the distal end of a 
lava stream frequently terminates in a nearly vertical escarpment 20 
to 30 feet or more high. Each of these variations is admirably illus- 
trated about Black Butte, in the valley of Big Wood River, where in 
places the lava advanced onto a plain and terminated in a feather edge 
of pahoehoe; again, to the northeast of the butte, the side of a lava 
flow when seen from below appears as a rugged wall of angular blocks, 
about 30 feet high. From a comparison of numerous examples which 
have come under ni} T notice it appears that streams of acid lava usually 
terminate in steep frontal slopes, while basic lavas, if in the condition 
to produce pahoehoe, have thin margins, and if aa is present, form 
rugged bordering slopes. 
MARGINS OF LAVA TERMINATING AT THE MOUNTAINS. 
Where the great southward-flowing lava stream from the Cinder 
Buttes met the deeply indented border of the mountains, interesting 
topographic changes resulted. The lava, as previously stated, was 
highly fluid when extruded, and flowed with almost the freedom of 
water. On the plain it spread out and formed what may be termed a 
lake of liquid rock. The western shore of this lake for a distance of 
fully 30 miles in a straight line is formed by steep mountain slopes 
and presents the general features that would be seen along the shore 
of a lake occupying the Snake River Plains. The margin of the lava 
is approximately a contour line. The difference in elevation of the 
surface of the lava in a distance of 30 miles is about 1,000 feet. Prob- 
ably considerably more than one-half of the descent occurs in the first 10 
miles after leaving the Cinder Buttes. In the southern two-thirds of the 
flow no eye can observe that it is not a perfect plain. The liquid rock 
entered the mouths of the valleys in the mountains which open out 
into the Snake River Plains and extended up them various distances, 
