TOPOGRAPHIC FEATUEES OF THE DESEET 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 

 do so at all. 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 Lliddle 

 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 siu-face 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. 



