TOPOGRAPHIC FEATURES OF THE DESERT BASINS. 9 
is beyond the scope of the present report, and which it is desired 
to avoid. It is thought best, therefore, to designate this period 
simply by the name of the great lake which best illustrates its 
history, and to refer to it as the Lahontan period. This is meant to 
include the whole period of deciphered lake history from the initial 
rise to the end of the second or final desiccation. No implication is 
intended as to the internal character of this period, and no specific 
names are applied to its various divisions. 
THE UNDRAINED AREAS OF THE UNITED STATES. 
It has already been noted that the Great Basin is not a unit. Its 
parallel mountain ranges cut 1t into numerous more or less connected 
valleys, and about halfway across the basin from east to west is one 
range in particular—the White Pine-Ruby Range—which has formed 
a major parting of the waters of the basin. East of this range is the 
Bonneville Basin, whose deepest depression was occupied by the 
ancient lake of that name and whose valleys now drain to its rem- 
nant—the Great Salt Lake of Utah. West of the range the Hum- 
boldt River cuts across the northern ends of the north-south ranges 
and discharges into the Carson Sink, once the home of the ancient 
Lake Lahontan. The basin of this lake then included not only the 
drainage of the Humboldt River, but also that of the Carson, Truckee, 
and Walker Rivers, the two latter of which have since been cut off 
by desiccation. These, with various smaller basins tributary to the 
early lake, form the Lahontan Basin. 
North of the whole of the Great Basin and south of the eastern 
or Bonneville section of it the ranges and trough valleys which char- 
acterize it merge into wide, dissected plateaus, that of the Columbia 
and Snake Piver lavas on the north and that of the Colorado Plateau 
on the south. The southern limit of Lahontan is very different. 
The great trough valleys which characterize the core of the Great 
Basin are diverse in their slope, some draining northward and some 
southward. Most often, however, they drain both ways from an 
alluvial divide somewhere near the center. Thus the troughs forming 
the eastern part of the Lahontan Basin drain into the Humboldt 
River from their northern portions, while their southern extremities 
slope and drain either toward smaller basins also inclosed or toward 
some tributary of the Colorado River. Farther to the west the south- 
ern boundary of Lahontan is a transitional area of irregular cross 
uplift in which are a number of small basins, conveniently grouped 
with those of the Nevada trough valleys that chance to be inclosed. 
West and southwest of these is the great trough system of California, 
containing the Owens, Searles, and Panamint Valleys and their 
smaller analogues, and the great basin of Death Valley, to which 
19750°—Bull. 54—14 2 
