'280 REPORT UNITED STATES ENTOMOLOGICAL COMMISSION. 
» 
tween the Niobrara and White Rivers, and the sand hills on the latter 
si ream, form the only exceptions wort hy of note, and indeed it is doubt- 
ful whether they should be excepted. The Llano Estacado is mainly 
covered with fine grass, while Jornada del Muerto, of New Mexico, de- 
rives Ls name, not from having the characteristics of a desert, but 
solely from the dearth of water. 
The Black Hills are an isolated group of mountains, some 75 miles in 
length by 50 in breadth, rising to a height of 6,000 to 7,000 feet above 
the sea, and about 3,000 feet above the surrounding level. They are 
heavily timbered throughout, while about their base the grasses are 
everywhere Luxuriant. 
The Mountain Area~ 
With regard to the Rocky Mountain region of British America, it may 
be briefly dismissed, as little is known about it in detail. It is essen- 
tially a heavily timbered region. It is made up of a number of ranges 
trending parallel to one another and separated by narrow valleys. It 
is a region of heavy rainfall and slight evaporation ; and not only the 
mountains>but the valleys also are covered with forest s. 
Within the Western United States the presence or absence, and even 
the comparative density and the prevalent species, of the forests may be 
predicted with a reasonable degree of certainty and detail from the lati- 
tude and elevation, i. e., from the same elements as there determine Hie 
degree of moisture of the climate. Excepting in the northern part of 
Idaho, Washington, Montana, and Wyoming, the general level of the 
country is not sufficiently elevated to cover it with forests, and conse- 
quently they are, except in the localities summarized above, confined 
to mountain ranges and high plateaus. In southern latitudes, eveu, many 
ranges of considerable altitude are not sufficiently high to reach the 
lower level of timber, as in the case of the majority of the ranges in 
Nevada, Arizona, and Southern California. 
Occupying the next zone below the forests, we find the grasses. They 
are found on the foothills of the mountains and the margins of the val- 
leys, and, in cases where the elevation and latitude are sufficiently high 
to insure moisture enough, covering the valleys. In other cases a zone 
of sage succeeds, the two growths grading into one another, and this in 
turn in the most arid localities, as in the deserts of Utah, Nevada, and 
Southern California, is succeeded by scanty growth of cacti, yucca, and 
by naked soil without vegetation. This succession, being a direct result 
of the conditions of aridity, can be premised regarding a certain region 
with considerable certainty. 
The more northern section of the Rocky Mountains in the United 
States, including the mountain region of Montana, Idaho, and Wyom- 
ing, is characterized, in the northern part, by a tolerably great precipi- 
tation, allowing a growth of forests almost as great as in the British 
Possessions. The whole of Kootenai County, Idaho, is covered by 
