58 SNAKE KIVER PLAINS OF IDAHO. [bull. 199. 
seolian origin from base to summit and clothed with vegetation. 
Although black and rugged and not yet dusted over with sufficient 
soil to support vegetation, except in the cracks and smaller depressions, 
the age of the lava stream is certainly considerable as measured in 
years. Big Wood River, after being displaced by the lava stream, 
continued to flow after the lava cooled, and throughout its length has 
cut a steep-sided canyon which is, in general, from 20 to 40 feet deep." 
The walls of this canyon are composed for the most part of hard, 
columnar basalt, and its excavation must certainly have required 
several centuries. This is interesting, when taken in connection with 
the absence of soil on the lava, as illustrating the slowness with which 
the soil of the Snake River Plains was accumulated. Extensive lava 
flows, the boundaries of which can not now be definitely traced, were 
poured out by the two older volcanoes just referred to, and the man- 
ner in which they are crossed and in part buried by the recent flow 
serves to show the maimer of formation of the Snake River lava plains, 
into which the region here referred to merges by insensible gradations 
and of which it is, in fact, a part. 
MARTIN LAVA STREAM. 
The descent of lava streams from the mountains bordering the Snake 
River Plains and the expansion of the lava so as to form a part of the 
plains themselves are again illustrated b}^ a flow of molten rock, which 
welled out from two openings in mountains of quartzit'e in the region 
drained by Lava Creek, about 5 miles southwest of Martin post-office, 
in Blaine Count}\ Martin is about 60 miles west of Blackfoot, on the 
west border of the Snake River Plains in their widest part. 
In the bold mountains, at the locality referred to, an exceptional 
feature in the relief is furnished by a steep conical elevation from 600 
to 700 feet high, composed principally of basaltic scoriae, the weathered 
sides and summit of which are conspicuous on account of their reddish 
color. At the base of this butte, on the northern and eastern sides 
and at an approximate elevation of 500 feet above-the level of the adja- 
cent portion of the Snake River Plains, two streams of lava were 
poured out, which cascaded down the steep slopes below and united to 
form a trunk lava stream about 400 yards wide, which flowed north- 
«An interesting feature connected with the erosion accomplished by Big Wood River is the pres- 
ence in the walls of its canyon at a locality about 2 miles northeast of Black Butte of characteristic 
potholes. These potholes, of which a score or more Avere examined, are from a few inches to 10 feet 
or more deep, and in the larger examples about 20 inches in diameter. They have been excavated 
in compact basalt by the grinding of sand and pebbles, caused to rotate by a swift current, and have 
smooth, nearly vertical walls. The only peculiar feature presented by them is the presence in cer- 
tain instances of central conical elevations at the bottom from a few inches to fully a foot high, 
which have spiral grooves on their sides. On looking down into a pothole exhibiting this feature, the 
seemingly spirally twisted elevation at the bottom has the appearance of the end of a large screw. 
So far as 1 am aware, the only other observed remnants of cores cut by revolving stones during the 
formation of potholes have been noted by G. K. Gilbert, Explorations and Surveys West of the one 
hundredth Meridian, Vol. Ill, Washington, 1875. p. 73. 
