168 J. LeConte on the great Lava-flood of the West, 
lutely universal flood, beneath which the whole original face 
of the country, with its hills and dales, mountains and valleys, 
lies buried several thousand feet. It covers the greater portion 
of northern California and northwestern Nevada, nearly the 
whole of Oregon, Washington and Idaho, and runs far into 
Montana on the east and British Columbia on the north. Its 
eastern and northern limits are not well known, but its extent 
cannot be less than 200,000 to 300,000 square miles, 1 e, 
greater than the whole area of France, or nearly double the 
area of California. 
Source—This immense mass of liquid matter was derived 
from streams which issued, as I believe, from fissures ; some of 
them in the Coast Range, but mostly in the Cascade and Blue 
Mountain Ranges. The streams from these two latter sources 
especially flowed until they met and formed an almost unt- 
versal sheet. 
ickness.—The greatest eruptive activity seems to have been 
in the region of the Cascade Range, and here therefore the flood 
seems to have reached its greatest depth. The area covered by 
the Cascade flows alone cannot be less than .00,000 square 
miles and the extreme thickness is not less than 8700 feet. The 
average thickness over the whole area is probably 2,000 feet. 
s this seems an extraordinary statement, we will briefly give 
the evidence on which it rests. 
contin- 
uity of the planes of the lava layers from to peak, surely 
4,000 feet is a moderate estimate for the original thickness ©: 
the lava flood at this 
