IRRIGABLE LANDS OF THE VALLEY OF THE SEVIER. 135 
Valley is composed of alluvial slopes; or, as they have been termed by 
geologists, alluvial cones. Although these surface features are presented in 
a somewhat more typical and striking manner in Grass Valley, yet they 
are well enough exhibited here; and as they have an important relation to 
the subject, I will briefly discuss them. 
In a mountainous country like this, where the melting of the snows in 
spring or heavy rainfalls at other seasons create sudden and great torrents, 
large quantities of detritus are carried down trom the mountains into the 
valleys. These mountain streams, which in summer, autumn, and early 
winter are ordinarily either very small or wholly dried up, may upon certain 
oceasions become devastating floods. The bottoms of the ravines are steep 
water courses, down which the angry torrents rush with a power which is 
seldom comprehended by those who dwell in less rugged regions. Huge 
» several tons, great trees, with smaller débris of rocks, 
boulders weighing o 
eravel, sand, and clay, are swept alone with resistless force, until the 
decreasing slope diminishes the energy sufficiently to permit the greater 
boulders to come to rest, while the smaller ones are still swept onward. 
The decrease of slope is continuous, so that smaller and smaller fragments 
reach a stable position, and only cobblestones, grayel, or sand reach the 
junctions of the streams with the main rivers. Around the openings of 
the greater gorges and ravines the deposits of coarser detritus build up in 
the lapse of time the alluvial cones. As it accumulates, each torrent builds 
up its bed and constantly changes the position of its channel, and with the 
mouth of the ravine for a center it sweeps around from right to left and left 
to right like a radius, adding continually, year after year, to the aceumula- 
tions of detritus. Thus a portion of a flat cone is formed, having its apex 
at the mouth of the ravine. At the foot of mountain ranges these alluvial 
cones are formed at the mouth of every ravine, and are sometimes so near 
together that they intersect each other, or become confluent. They are 
composed of rudely stratified materials, ranging in size or grain from fine 
silt and sand to rounded stones of several hundredweight, and occasionally 
a block of a ton or more may be seen near the apex of the cone. In 
regions where the rocks are soft and readily disintegrated the stones are 
