354 JW. Powell's Survey of the Rocky Mountain Region. 
cene) has been subject to a great and continuous erosion. The 
district of the High Plateaus occupies a portion of a narrow 
belt separating the Plateau Country from the Basin Province 
and therefore stands upon the locus of the ancient shore line 
which in the lacustrine stage bounded the two areas. To that 
shore line they stand in an intimate and remarkable relation. 
To its trend the great displacements maintain not merely gen- 
eral parallelism but an approximation to strict parallelism both 
in totality and in detail which would not have oie anticipated 
and which cannot be purely accidental, and seems to point to 
some definite determinative association between the littoral 
from the arched, flexed and tilted types prevailing in other 
disturbed localities. There is an abrupt transition from this 
preserved. 
But of all the features displayed by the High Plateaus the 
most remarkable are the manifestations of former volcanic 
activity. Both in area and thickness the volcanic emanations 
ve. They cover more than 5,000 square miles, 
‘SS & 
ter pa 
lace after the lake basin had been ned or had shrunken to 
imits outside of the district, for sedimentary beds have not been 
found intercalated between the various flows but always under- 
he them. It is therefore impossible to fix with great precision 
the commencement of the outbreaks, but the general indications 
Neericey, 
