170 J. LeConte on the great Lava-flood of the West, 
lava. Over the whole plain the lava cannot be less than 1,000 
feet thick. Beyond the Des Chutes plains and separated from 
them by a lava ridge, stretches, for twenty miles more, to the 
base of the Blue Mountains, the region drained by the tributa- 
ries of the John Day River. Here the flows from the Cascades 
meet and mingle with those from the Blue Mountains, and both 
with /ocal fissure eruptions. The lava in this region is less 
thick, only about 600 to 700 feet ; and being underlaid by the 
remarkable fossiliferous Miocene lake deposit of the John Day 
Valley, erosion has cut through the lava cap into the soft strata 
beneath, giving rise to that extraordinary jumble of steep 
round hills, some capped with lava and some uncapped, called 
in the expressive vernacular, “the devil's potatoe patch.” 
Imagine a patch twenty miles square, thickly covered with pota- 
toe hills 1,000 feet high, and we have some idea of the extraor- 
dinary appearance of this region, as seen from the dividing ridge 
mentioned above. 
Beyond this region I have not followed the lava flood; but 
others have traced it over a large part.of the basin of the 
Columbia and especially of the Snake River. Clarence King 
traced it for 300 miles along the Snake River, where it is 700 
feet thick. 
IL Structure of the Cascade Mountains. 
The Cascade Mountains, in the region of the Columbia River, 
with the exception of a very small portion at its very base and 
in its axis, is composed wholly of lava layers piled one atop 
another to a height of nearly 4,000 feet. Through this 1m- 
mense mass of lava, the river, whether in the form of ice-stream 
or water-stream, or both, has cut its way for 100 miles nearly 
to the sea-level, forming a cafion of unsurpassed magnificence. 
At the cascades of the river, which is in the very axis of the 
range, the base of the lava has been reached for a little dis- 
tance, and the original surface upon which the lava was outpoure 
is revealed. Examination of the sub-lava material fixes, I think, 
with more certainty than has yet been done, the date of the 
Cascade lava-flood, and, therefore, the age of the Cascade 
po 
In August, 1871, under the guidance of Mr. Condon, I first 
examined the magnificent section of the Cascade Mountains 
short ratirosd on the northern or haa tate side. The river 
current washes strongly against the south wall of the cafion, 
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