24 BULLETIN 54, U. S. DEPARTMENT OF AGPJCULTUEE. 



ing basins or into the Columbia River. In many cases it is now impossible to determine 

 tbe original direction of drainage of these pans and both for this reason and because of 

 the inadequacy of the available maps, the divides have been only very roughly deter- 

 minable and the calculated area is very approximate indeed. This applies particu- 

 larly to the Great Sandy Desert, across which passes the northern boundary of the 

 Clmstmas Lake Basin. This is a region of very low relief. The slopes are usually not 

 determinable except by precise leveling and most of the drainage channels Avhich 

 once existed have been wholly or partly obscured by dune sand and alluvium. 



The present floor of the Christmas Lake Valley is a broad plain, apparently fiat and 

 diveisified by several dune areas, especially in its eastern part. It rises more or less 

 gently to the surrounding rolling plateau and shows none of the usual features of lake 

 or playa topography. It is quite possible that it had once an outlet reaching from its 

 northwest corner through the Fort Rock Valley and the Deschutes River to the Coltim- 

 bia. This region has never been mapped and was not carefully examined by the 

 writer. The question must be left open, though the assumption of recent and reason- 

 ably free outlet would explain the absence of playa or lake traces and the general topo- 

 graphic resemblance to a tributary rather than a receiving valley, matters which are 

 difficult to understand on the assumption of continuously inclosed conditions. 



Christmas and Fossil I^akes, with several other small playas or marshes now present 

 on the valley floor, are mere local depressions formed by wind erosion or dune move- 

 ment, or both, and fed by springs or local drainage. Christmas and Fossil Lakes owe 

 their comparative permanence to supply from springs. Neither has any relation to 

 the earlier topography of the valley. Thorne Lake, in the southwestern corner, is a 

 small enlargement and local depression in the channel through which the overflow of 

 Silver Lake once passed into the Christmas Lake Valley. . 



At the present time there is no area of considerable drainage concentration in the 

 valley. Peter Creek, rising in the southern slopes of the Pauline Mountains, maintains 

 a well-defined channel for some distance southward, but finally loses its water to the 

 underflow without forming a lake or playa. Christmas and Fossil Lakes receive the 

 drainage of their immediate surroundings only. During the Lahontan period the 

 drainage area was about 2,000 square miles, exclusive of the Silver Lake Basin, next 

 described. Including this, the area was about 2,750 square miles. Because of the 

 difficulty of determining the actual position of- the limiting divide, the figure for the 

 Christmas Lake Valley proper is scarcely more than a rough approximation. 



THE SILVER LAKE BASIN. 



Silver Lake lies southeast of the Christmas Lake Valley, in a basin of structural 

 origin and bounded by lava scarps and slopes in the manner typical of the region. To 

 the west and southwest its drainage reaches to the crest of the lava plateau and a 

 somewhat indefinite parting from the headwaters of the Klamath River drainage. 

 Between it and the Christmas Lake Valley is the small but steep local uplift of the 

 Conley Hills and Table Pvock. There are several low gaps in this upUft, and one of 

 them, south of Table Rock, is only a few feet above the present Silver Lake and con- 

 tains a dry channel through which Silver Lake discharged into the Christmas Lake 

 "\'alley very recently indeed. The present Silver Lake occupies the southern portion 

 of its valley, the northern portion being occupied by the Pauline Marsh, which empties 

 southward through the Pauline Slough. The lake is very shallow but practically fresh, 

 a fact which is accounted for by the recency of overflow. The lake is somewhat 

 variable in size and is reported to have entirely evaporated in 1886-87. The present 

 drainage area is essentially the same as that of the Lahonlan period, and is about 750 

 square miles. There is some uncertainty in the position of the divides in this area, 

 but the uncertainty is far less than in the case of Christmas Lake Valley. 



