34 GEOLOGY OF SW. IDAHO AND SE. OKEGON. [bull. 217. 
spring it becomes covered with a few inches or possibly 1 or 2 feet of 
water and transformed into a lake, which when fully expanded covers 
an area of between 50 and 60 square miles. The water supply for 
this winter lake is furnished principally by the small streams which 
descend the precipitous east face of Stein Mountain, but a small con- 
tribution is made by a hot spring on its southwest border. In sum- 
mer the desiccated bed of the lake is a smooth, even, hard mud plain, 
crossed in every direction by a rather fine network of small shrinkage 
cracks. Scattered over the smooth and frequently glossy surface 
there are, at intervals usually of several rods, fragments of rock, some 
attaining a diameter of perhaps an inch. The manner in which these 
angular fragments reached their present resting place is obscure, but 
it is probable that they were transported and deposited by floating 
ice. Frequently on clear, hot summer days the delusive effects of 
the mirage relieve the desolate monotony of the desert and the sur- 
face is seemingly flooded with water. To the south of Alvord Desert 
and in the same valley or basin is another and smaller playa lake, 
known as Alvord Lake, which is supplied during the wet season by 
Trout Creek, and also receives the waters from a constantly flowing 
hot spring. 
North of Alvord Desert the valley adjacent to the east base of 
Stein Mountain becomes narrow, its general width being about 2 miles, 
and the streams from the bordering mountains have deposited large 
alluvial cones, some of which extend entirely across the depression 
and meet the debris on its eastern side. In this manner the valley 
has been divided into a number of shallow basins, four of which hold 
small playa lakes, of which Juniper and Mann lakes are representa- 
tive examples. 
The basin in which Juniper Lake is situated was formerly more 
deeply flooded, as is shown by beach lines about its borders, and con- 
tained a lake about 7 miles long, 2 to 3 miles wide, and 100 feet deep. 
About the little valley in which Mann Lake occurs there are also 
faint beach lines which show that it formerly contained a lake 
approximately 150 feet deep and about 4 miles long and 2 miles wide. 
Several of the small basins between the sharp-crested ridges to the 
east of the two lakes just mentioned are occupied by mud flats in 
summer and by ephemeral lakelets in winter. a 
FLUCTUATIONS OF LAKES AND EFFLORESCENCES. 
The lakes of Harney and Alvord valleys, and other places described 
above, are similar in many ways to a large number of lakes scattered 
a As the facts concerning a few of the inclosed lakes of Oregon here presented may perhaps 
lead the reader to wish to continue the study further, references are given to some of the more 
accessible reports, books, etc., bearing on the subject: 
Russell, I. C, Geological history of Lake Lahontan: Mon. U. S. Geol. Survey, vol. 2, 1885. 
Russell, I. C, Quaternary history of Mono Valley, California: Eighth Ann. Rept. U. S. Geol. 
Survey, 1889, pp. 261-394. 
Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1, 1890. 
Russell, I. C, Lakes of North America, 1895. 
