288 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
whi< h lie to the eastward separating it from the valley of the Blackfoot. 
This upper valley, after being abandoned by the stream, continues 
on southward to the Bear, where it is known as Gentile Valley, and of 
which we have spoken above. 
Passing through the Portueuf Range, the Portneuf enters, near its 
lower end, a broad, fine valley, occupied by Marsh Creek, the most im- 
portant tributary of the river. This valley heads opposite that of the 
Malade, and extends, with a gradually decreasing width, 28 miles north- 
ward. Its greatest width is 12 miles. Bench land forms the greater 
part of the valley, and this produces mainly sage, with a small admix- 
ture of grass. Marsh Creek, the small stream which flows through this 
valley, has a marshy bottom laud through most of its course, from one- 
fourth of a mile to a mile in breadth, which produces marsh grasses and 
willows. All the valley can be burned over, as well as the lower slopes 
of the Portneuf Range on the east and of the Bannack Range on 
the west. These ranges contain but little timber, and that near their 
crests. 
West of the Bannack Range are the valleys of the upper waters of the 
Little Malade and of Bannack Creek, both open and grassy, with more 
or less sage, and both easily burnable. 
At the foot of Marsh Creek Valley the Portneuf turns to the west for 
a few miles, cutting its way through a mass of high hills, then turns 
northwest, and, the mountains falling away on either hand, the river 
sweeps out into the Snake River Plains, in which it joins the Snake 
River. 
The Snake River Plains are an enormous field of basalt extending 
westward from about longitude 112° nearly to the western boundary of 
Idaho, and from near latitude 42° uorth to the southern base of the 
Bitterroot and Salmon River Ranges. The surface is slightly undu- 
lating and is seamed with crevasses like a field of old ice. Most of the 
streams which enter this region soon disappear beneath its surface, 
perhaps to appear and disappear again. The soil is mainly a shifting 
sand, which, driven by the prevailing westerly winds, has collected in 
dunes ou the eastern and northeastern border. This great area is mainly 
covered with sage, which grows luxuriantly, attaining arborescent pro- 
portions. In the interior and southern portions of this waste this mam- 
moth growth of sage is the only product of the soil, but near the base of 
the mountains on the east and north grass gradually takes the place of 
sage, in a measure, and on the lower mountain slopes it monopolizes the 
soil to the practical exclusion of other growths. 
The country along the northern margin of these plains, i. e., that lying 
at the base of the Bitterroot aud Salmon River Ranges, with the lower 
slopes of these mountains, can easily be burned over. The larger part 
of the area of these plains, however, falls in that debatable ground 
where it is very difficult to decide whether it is or is not burnable, eco- 
nomically. In some localities the Artemisia is so abundant and so luxu- 
