140 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The western face of Jackson range near its northern end had the same ap- 
pearance, but this was very imperfectly seen. Further details concerning 
the first two of these ranges are given in a later section. Examples of 
sloping waste plains in front of dissected ranges are given in Plate 2, 
while the linear front of the Pueblo range is illustrated in Plate 3, a. 
The Base Line of Residual Mountains. —It may be worth while to 
state at this stage of the discussion the reasons for rejecting the theory 
that the mountain ranges just described are the residuals of much 
larger masses, of which the vanished parts have been removed by ero- 
sion. These reasons are found, not at al] in the incompetence of erosion 
to wear away mountains, but in the impossibility of explaining the 
forms of the mountain ranges above-named as the residuals of much 
larger masses. There are numerous examples in which the general sub- 
aerial erosion has sufficed to remove mountains more or less completely, 
but no examples in which the residuals of half-consumed mountains 
exhibit the features above described as characteristic of certain Basin 
ranges. Several special cases may be considered. 
The only residual mountains known to physiographers as having a 
relatively continuous mass and rectilinear base are those in which struc- 
ture controls form, as in the stratified Appalachians of Pennsylvania 
and Virginia. There the ridges of resistant sandstone rise between 
rolling lowlands of weaker strata; the ridges are occasionally cut 
through in water gaps, but between the gaps they frequently present 
a continuous mass slceping evenly to a nearly rectilinear base. When 
the strata bend, the ridges turn: when the strata are cut off by a fault, 
the ridges end. Structure is perfectly expressed in form. The same 
rule applies to the trap ridges of the Triassic areas of Connecticut, New 
Jersey, and Pennsylvania; but the rule clearly enough does not apply 
to the Wahsatch mountains and the other Basin ranges above named. 
Residual mountains whose survival is not dependent on contrasts of 
rock resistance so striking as those of the Appalachians, and whose 
structure is relatively massive, are well illustrated in the crystalline 
Appalachians of North Carolina and Georgia. In mountains so old as 
these, it is to be presumed that the valleys have generally come, by a 
process of long-sought adjustment, to follow the somewhat weaker 
rocks, while the mountains represent the more resistant masses. None 
of these mountains, however, have a bold descent to a nearly rectilinear 
base; all of them give forth spurs which as a rule slope more and more 
gradually as they fade away on the valley lowlands; while branch 
valleys enter between the spurs far into mountains. The mountain 
base line is sinuous and ill-defined. 
