TOPOGRAPHIC FEATURES OF THE DESERT BASINS. 31 
to two tributaries of the Klamath group—the Klamath and Sycan Marshes. During 
the Lahontan period the whole of this region doubtless drained freely to the sea, but 
subsequent desiccation has so weakened its streams and increased fan-building that 
much of the area is now cut up into small basins and local saline playas. Even the 
large Tule Lake overflows only intermittently and several of the smaller lakes do not 
dosoatall. All this, however, is quite recent and essentially the region has been and 
is one of seaward drainage. 
THE TROUGH VALLEYS OF NEVADA AND THE BASINS OF THE TRANSITION ZONE. 
The general character of these basins and their relations to the 
other divisions of the Great Basin were noted briefly on page 9. 
There is really no essential structural difference between them and 
those similar trough valleys which have chanced to drain to Lake 
Lahontan or to the Amargosa River, but this difference of drainage 
is quite important from the present viewpoint and makes desirable 
a separate treatment. The valleys of this division, though much 
alike in essentials of structure and topography, present an almost 
infinite variety of detail. It is obviously impossible to discuss 
them thoroughly, and the following statements are confined to a brief 
note of location and to those facts essential to the present inquiry. 
THE DIXIE BASIN. 
The Dixie or Osobb Valley occupies the first inclosed trough east of the Carson Sink. 
It now receives the drainage of the Pleasant Valley from the north and the Middle 
Gate and East Gate Valleys from the southeast. Neither of these drainage lines is 
now active, but both are freely open and are still traversed by the flood waters of 
heavy storms. The Fairview Valley to the south was probably once a tributary 
of the Dixie, but is now cut off by a low ridge the nature of which is not fully certain. 
The writer regards it as probably due to recent minor faulting, but possesses no con- 
clusive evidence to this effect. Behind this barrier has been formed a small nonsaline 
playa known as Labou Flat. 
The northern end of the Pleasant Valley is separated from the Humboldt Valley 
by a divide the present surface of which is alluvial, but this divide is high above 
the bottom of the Dixie Valley and the valley is believed never to have discharged in 
this direction or in any other. The greatest depression of the valley contains a mud 
flat nearly 60 square miles in area, in the center of which is a body of loosely crys- 
tallized common salt about 10 square miles in area and from 2 to 10 feet thick. 
This salt deposit is known as the Humboldt Salt Marsh and was once the source of 
commercial salt for metallurgical purposes. Old strand lines 150 feet and 40 feet above 
the present surface of the salt bed indicate the existence and fluctuations of the lake 
from which it was probably derived. 
The area now permanently or occasionally tributary to the Dixie Valley is 2,000 
square miles. The Fairview Valley has an area of 290 square miles, making a total of 
2,290 square miles for the probable Dixie Valley of Lahontan time. 
THE GABBS VALLEY. 
Gabbs Valley lies southeast of that last described and is the northernmost of the 
small basins which constitute the transition zone. It is entirely surrounded by 
mountains of considerable height and has almost certainly never overflowed. It 
contains a saline flat with a sandy instead of a mud surface and about 25 square miles 
in area. There are no traces of an early lake. The total area of the basin is 1,280 
square miles. 
