130 SNAKE KIVEft PLAINS OF IDAHO. [bull.199^ 
ried away, but are engulfed in the stratum beneath, space being fur- 
nished by the removal of the tine material by the spring. 
In the process now going on in the enlargement of the Blue Lakes 
alcove, etc., a portion of a thick stratum of sand is being replaced by 
a deposit of angular blocks of lava. Possibly the mode of origin of 
certain ancient breccias may here be indicated. 
The spring-formed alcoves on the side of Snake River Canyon ard 
unique topographic features, because the combination of conditions 
necessary for the production of such recesses or lateral cuts in the 
border of a plateau is of rare occurrence. 
LOST RIVERS. 
In connection with the following brief account of a striking pecul- 
iarity in the hydrography of the Snake River Plains, the reader is 
invited to refer to the map forming PI. I. As there indicated, not a 
single tributary reaches Snake River from the high and rugged moun- 
tains lying west of its course between Malade River and Henrys Fork, 
a distance in a straight line of 200 miles. Several of the eastward- 
flowing streams which rise in the mountains referred to are of consid- 
erable volume and reach the margin of the Lava-covered plain bordering 
Snake River, but fail to cross it. Examples of such streams, named j 
in order from northeast to southwest, arc Camas, Heaver, Medicine 
Lodge, and Birch creeks and Little Lost and Big Lost rivers. When 
the snow is melting each of these streams advances onto the plain for 
distances varying, in the several cases cited, from 10 to 50 miles, but 
shrinks back in summer and ends indefinitely soon after leaving the 
valley tracts. In certain instances, as in the case of Camas and Birch 
creeks and Little Lost and Big Lost rivers, the waters spread out oj 
the marginal portion of the plain during the period of their greatest 
elongation and form shallow lakes. These lakes are ephemeral, how- 
ever, and although renewed each spring, with one exception become 
dry each summer, leaving smooth mud plains or playas. The excep- 
tion is furnished by Camas Creek, which expands to form Mud Lake, 
which fluctuates in area from month to month and when largest is 
from 40 to 50 square miles in area. It was dry in the summer of 1891 
and lower during the summer of 1900 than at any time during the 
preceding nine years. In September, 1900, the bed of Camas Creek 
was dry for a distance of 20 or 30 miles adjacent to its distal extremity, 
although, as indicated by a well, there was an underflow beginning at 
a depth of 20 feet. Mud Lake was then shrunken so as to leave a mud 
flat from a few to several miles broad about its borders. 
The streams just referred to emerge from valleys deeply filled with 
alluvial material and during high-water stages bring to the plain large 
quantities of material held in suspension or rolled along their bottoms. 
The coarser portions of this, the ''visible loads" of the streams, are 
