60 SNAKE RIVER PLAINS OF IDAHO. [bull. 199. 
lava streams unite with the still more extensive sheets of similar rock 
along the border of Snake River without any marked topographic or 
other contrast to indicate that they are not portions of a single series. 
As will be shown below, lava sheets originating from volcanoes in the 
broad central basin in certain instances entered the mouths of lateral 
valleys and extended up them, as the waters of a lake would do, for a 
score or more miles. The direction from which the lava in the lateral 
valleys flowed can in many instances be readily determined and the 
extent of the individual lava sheets ascertained and mapped, but this 
does not seem to have been attempted by the survey referred to above. 
The most definite and, as it seems to me, the most accurate statement in 
reference to the sources of the Snake River lava to be found in geolog- 
ical literature occurs in the text of the Boise folio, by Lindgren." He 
says: "The basalts were erupted from a great number of inconspicu- 
ous craters, both in the plains and in the adjoining mountains. Their 
fluidity was remarkable, continuous flows of 50 miles or more being 
noted. One flow, for instance, followed the South Fork of Boise 
River for that distance down to its mouth." The lava just referred' 
to was seen by me, and there is no doubt that after flowing fully 50 
miles as a stream from 1 to 3 miles wide, and in general about 300 
feet deep, it retained sufficient fluidity after emerging from the valley 
through which it descended and entering the Snake River Plains to 
spread widely. This lava stream since it cooled and hardened has 
been dissected throughout its length by the river it displaced, leaving 
flat-topped terraces at numerous localities. 
The sources from which the Snake River lava came are, as stated by 
Lindgren, in part in the mountains bordering the plains and in part 
on the plains themselves. It will be instructive, I think, if the vol- 
canoes furnishing these two sources of supply be considered separately. 
VOLCANOES AMONG THE MOUNTAINS. 
The fact has already been mentioned that a portion of the lava now 
covering the Snake River Plains came as a narrow stream which 
descended the valley of the South Fork of Boise River for fully 50 
miles and that other and larger streams probably originated in the 
craters near Soda Springs. These examples need not be considered 
further, as I have no information to present concerning them in addi- 
tion to what has already been referred to. 
BLACK BUTTE. 
The fact that volcanoes situated in the mountains bordering the 
Snake River Plains poured out lava in much the same manner as 
«See also papers by the same author in Eighteenth Ann. Kept. U. S. Geol. Survey, I't. Ill, pp. 635- 
636, and Twentieth Ann. Kept., Pt. II, pp. 99-100. 
