142 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
being abruptly terminated at a well-defined base line, as is so persist- 
ently the case with the above-described ranges of Utah and Nevada. 
In some cases the spurs run far forward, forming ridges of undulating 
outline by which embayments of the piedmont lowland are divided. 
The contrast of the Blue Ridge escarpment and the Basin ranges is 
therefore extremely instructive. The topographical maps of the North 
Carolina mountains are worth examining in this connection: Wilkes- 
boro, Cranberry, Mt. Mitchell, and Pisgah (N. C.), and Hillsville (Va.) 
sheets afford the best illustrations. Some account of their peculiar 
features is given in a recent essay by the author (1903). 
The only conditions under which residual mountains have a well- 
defined, moderately curved base line is where they are cut across by a 
master river, or laterally attacked by the waves of a vigorous sea; but 
these conditions are so manifestly inapplicable to the region of the 
Basin ranges that they need no consideration here, except in so far as 
they suggest that a trenchant cause is needed to explain the well-defined 
base line to which the ranges descend. 
Residual Mountains in the Great Basin. — There are, however, some 
excellent examples of residual mountains among the Basin ranges. 
Those that I saw are of much less height than the ranges thus far 
described. Their forms are thoroughly subdued. They have no well- 
defined and moderately curved base line, but descend in branching, 
sprawling, fading spurs, which interlock with broad, flat-floored, branch- 
ing valleys. The contrast of these nearly worn out mountains with the 
more vigorous forms previously considered is most striking, yet it is 
entirely conceivable that the contrast may be due simply to stage of 
development and not to difference of origin. It has already been shown 
that the late stage of dissection of a fault-block mountain would, long 
after faulting had ceased, present essentially such worn-out forms as are 
here described, for the sharp definition of the base line would be lost 
after faulting had weakened and stopped. On the other hand, the old 
residuals of massive mountains of any other kind would also present 
these worn-out sprawling forms. There are indeed no tests by which 
the two kinds of old mountains can be easily distinguished. 
Several examples of residual ranges in the Great Basin were noted as 
follows: North of Tecoma, Central Pacific railroad, there are mountains 
of moderate relief, whose rounded branching spurs descend gradu- 
ally to low, sprawling, dwindling terminals, between wide-open, waste- 
floored valleys. The fading spurs and open valleys interlock on a very 
sinuous line. These well-defined features gain an added value by their 
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