196 Geological Survey of the Territories in 1872. 
the stream. 
On reaching the Port Neuf, we first encounter the volcanic 
rocks which have filled the great plains of the Snake River 
Basin. The valley of the Port Neuf is floored with basalt, 
which escaped from one or more old craters near the bend 0 
ally flowed. 
‘As we reach the plains, we find them underlaid by from three 
to five layers of basalt separated by beds of sand and gravel, 
showing as many repetitions of the voleanie eruptions, occur 
ring after long intervals, during which intervals the mountaip 
streams brought down and. distributed widely over the plains 
immense amounts of well-rounded pebbles nef 
ainly of quartzites, limestones and metamorphic rocks, but 
including also some of the lavas themselves. Upheaval has 
gone on here until a very recent period ; since upturned porphy- 
ries and basalts, interlaminated with Pliocene sandstones and 
limestones, occur at many points along the foot-hills bordering 
the plains; but such disturbance seems to have now nearly 
or entirely ceased. a 
Reaching Fort Hall on July 38d, wagons were “ turned 10, 
a pack train was outfitted, fresh supplies were secured, and the 
party started again, on the 12th, across the broad lava plains . 
the northward, leaving for a time the Paleozoic rocks whic 
form the mountain ranges and which are overlaid (unconforma- 
