g2 The American Geologist. February, 1002. 
and partial decomposition of the bed rock extended to con- 
siderable depth, perhaps as much as several hundred feet 
on the average, softening the rocks and making them less 
resistant to subsequent weatherings and stream erosion. 
Subsequently to the uplift, after the andesitic covering had 
been removed, this comparatively easily eroded material 
would be carried away rapidly, while, when the streams 
cut down into the undecomposed rock, their work may have 
been much less effective and the results more inclined to- 
ward the cafion form. 
However, the contrast between the rolling uplands and 
the cafions @§§t00 great to be entirely accounted for under 
the abovesd mm sis. The rolling pea as along the 
met eat river, are undulating plains miles in extent 
ore suggesting a mountain region than does the 
zark plateau in Missouri. They lie at a level 
usually sev hundred feet below that of the flat-topped 
divides, which latter are remnants of the volcanic plain. 
The narrow strips of higher ground isolate them into broad 
shallow basins, which are sometimes referred to as the upper 
troughs of streams. In them the peneplain has been 
uncovered, d the present topography, except for the 
cafions, is probably similar to that of the late Tertiary time 
before the peneplain was buried. However, the surface 
at present is generally somewhat lower than that of the old 
peneplain. 
In Tuolumne county, as Ransome has pointed out,* 
there are lo@al dissected plains at the level of the rolling 
uplands which certainly postulate a base level of erosion not 
sufficiently accounted for by the presence of a resistant rock 
barrier down stream, and I am inclined to accept the opin- 
ions of others that the valley erosion of the Sierra Nevada re- 
gion was affected in two distinct periods whose products are 
respectively the upper troughs or rolling uplands and _ the 
cafions. 
It is thought probable that the Sierra Nevada region stood 
somewhat higher during the volcanic period than it did dur- 
of the 
*The Mother Lode District folio of the Geologic Atlas of the United States, 
page 7, 3rd column, 
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