bussell.] SOUECE OF THE LAVA. 65 
Several examples maybe seen from the trains of the Oregon Short 
Line while passing between American Falls and Mountain Home. 
One of the elevations on the plain between Shoshone and Shoshone 
Falls, known as Flat Top Butte, displays evidence of a former crater 
perhaps more conspicuously than usual. From the summit of one of 
the craters under consideration, situated about 20 miles north of 
Shoshone, between Little Wood and Big Wood rivers, I counted eight 
similar elevations, but others known to be present on the vast plain 
1 v i ng to the east were obscured by the desert haze. Several others were 
in view from the summit of Big Butte, on the plain to the west and 
south. The elevations referred to are so broad at the base, and so low, 
that when looking down upon them, as, for example, from the neigh- 
boring mountains, their presence might easily pass unnoticed, except 
when illuminated by a strong side light, although they are easily 
recognized from lower points of view, when seen in profile against 
the even sky line. 
Not all of the volcanoes that supplied the lava forming the Snake 
Riv r er Plains, however, are situated on their surface. Several craters 
which sent long streams of lava down tributary valleys have been 
mentioned by previous observers, and some of these were seen by 
myself. Several craters composed largely of scoriaceous basalt are 
know r n to exist in the valleys to the southeast of the Snake River 
Plains, as, for example, in Basalt Valley, Blackfoot Valley, etc., in the 
vicinity of Soda Springs. They have been described by the geologists 
of the Hayden survey. Extensive lava streams came from these cra- 
ters and followed the valleys, principally in a northwesterly direction, 
to where they open out into the broad Snake River Plains. Certain 
of the geologists referred to suggest that these lava Hows, which gave 
several extensive valleys level floors, reached the greater valley bor- 
dering the Snake and assisted in the formation of its present surface, 
but that this really occurred is left undecided. In this connection 
F. H. Bradley" states that the basalt bordering Snake River may have 
come in part from the crater near Soda Springs, but remarks: "It 
hardly seems possible that after flowing 70 or 80 miles the lava sheets 
could still have retained sufficient fluidity to be spread out in a solid 
layer over the plain." 
The craters and lava flows just referred to have not been critically 
studied by me, but what is known concerning them, when viewed in 
the light of other and similar occurrences, certainly suggests that the 
lava streams originating at the craters in the vicinity of Soda Springs 
and descending the valle3 r s occupied by the Portneuf and Blackfoot 
rivers, Willow Creek, etc. , may have retained sufficient fluidit} T on reach- 
ing the Snake River Plains to enable them to spread widely. These 
a Sixth Ami. Rept. U. S. Geol. and Geog. Surv. Terr., 'Washington, 1873, p. 201. 
Bull. 199—0:2 5 
