RUSSELL.] LAKES. 35 
throughout the Great Basin, but occurring principally in its northern 
half. The reason for the presence of more lakes in the valleys of the 
northern than in similar inclosed basins in the southern part of the 
Great Basin, is evidently due principally to a slightly greater mean 
annual precipitation and a slight decrease in evaporation as one 
journeys from the south toward the north in that region. The 
lakes of southeastern Oregon furnish an instructive contribution to 
the study of the delicate balancing of conditions referred to, and 
illustrate the manner in which mountains may become the controlling 
condition on which depend the existence of the lakes in neighboring 
valleys. For example, if mountains had not been upraised about the 
Harney Valley to act as condensers and give origin to streams, it is 
evident that the basins of that region would not have had lakes 
developed in their deeper portions. 
Another feature in which the portion of Oregon here considered is 
representative of the Great Basin in general is the deep filling in the 
valleys. In a region which has no outflowing streams, all the mate- 
rial brought from the bordering or included uplands and mountains 
by the rain wash, rills, brooks, etc. , is deposited in the valleys. For 
this reason the depressions normally become deeply filled with stream- 
deposited alluvium and with the sediments spread out in lakes. 
These two processes of deposition are frequently in operation at the 
same time, and it is extremely difficult in many instances to deter- 
mine from a study of the accumulations whether they were made by 
flowing or by still water. In general the valleys throughout the 
Great Basin have been deeply filled with fine silt-like material having 
a light-yellow color, deposited in part by streams and in part in lakes. 
The valleys have thus been given nearly level floors in their central 
parts, and on their borders slope upward in gentle curves to meet the 
bordering hills and mountains. This is the case in the Harney and 
neighboring valleys, and the surface deposits of fine silt form rich 
soil, which needs only water to make it highly fruitful. 
In a region devoid of outflowing streams all of the material brought 
to the valleys by streams in solution — their "invisible loads" — as well 
as all of the debris they carry in suspension — their visible loads — is 
left as the waters evaporate. Hence not only the lakes of such regions 
are characteristically alkaline, but the soils are apt to contain easily 
soluble salts. Within each inclosed basin there is an assorting and 
concentration of saline matter, and as frequently happens the lower 
lands are strongly alkaline. At times, as the waters evaporate, sur- 
face incrustations or efflorescences are formed, consisting principally 
of common salt, sodium sulphate, sodium carbonate, etc. , but in rare 
instances they also contain boracic acid, and are valuable as sources 
of borax. 
Several of the small playas in southeastern Oregon are covered in 
summer with white incrustations of the nature just described. A 
