50 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
the faulting, by which the first eycle was interrupted; and a second or 
post-faulting cycle of erosion, producing the forms of to-day. The 
Swazy and the Sawtooth masses are of special interest in these respects, 
since they both possess features that cannot be reasonably accounted 
for by post-faulting erosion, or by erosion in a single cycle uninter- 
rupted by faulting, and yet which are very reasonably accounted for 
by pre-faulting erosion. The features of the southern mass will be 
considered first. 
The oblique course of the scarp-making layers on the back slope of 
the Sawtooth mass has already been mentioned. The northernmost 
scarp, which overlooks the valley worn on the Trilobite shales, may be 
in some way related to the fault that is supposed to divide the two 
southern masses; the other scarps, especially the one formed on the 
uppermost gray limestone, are purely the work of retrogressive erosion. 
"Their course is only locally interrupted by the granitic intrusion and 
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Fic. 26.— Valley along the contact of intrusive granite and middle shales (?) in 
the highlands of the Sawtooth mass; looking west. 
by the valleys of the back slope. On the whole they maintain their 
oblique ascent of the range in rather regular order, as indicated in 
figure 17. The escarpment of the middle layers facing northward 
towards the granite is shown in figure 26. Three suppositions may 
be made in explanation of these features: —a single southeast-dip- 
ping monocline, long eroded in a single cycle, without further distur- 
bance by tilting or faulting; a southeast-dipping monocline, long ago 
cut in two by a north-south fault and the eastern block uplifted with 
an eastward slant before much erosion of the monocline had been 
accomplished; the same, but with subrecent faulting and uplift after 
much erosion of the monocline. The second and third cases differ 
from the first in involving two cycles of erosion, instead of only one; 
they differ from each other in that the second case postulates a short 
first cycle and a long second cycle, while the third case postulates 
a long first cycle and a short second cycle. 
