52 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
sion of the uplifted block. The peculiar consequences of the third 
supposition accord so well with the facts of observation that the sup- 
position is warranted. 
It must therefore be concluded that the district of the Sawtooth 
mass was reduced, in a cycle of erosion before the block faulting, to a 
series of monoclinal ridges and valleys of moderate relief, the general 
series of trends being about northeast and southwest, and the drainage 
being adjusted to the structure in longitudinal and transverse courses. 
Since the opening of the second cycle of erosion by the upfaulting of 
the mountain block, the fault face has been deeply carved and the back 
slope has gained a greatly increased relief in the incision of many val- 
leys, most of which seem to result from the revival of the preexistent 
adjusted streams; but the oblique course of the strata on the back 
slope still persists in the general pattern of the first cycle. 
One curious feature deserves special mention here. The knobs or 
teeth from which the Sawtooth range gains its name are formed of the 
Fia. 27.— The knobs of the Sawtooth mass, looking south, 
uppermost limestone, where its scarp reaches the crest of the block. 
Seen from the eastern side, the teeth are round and dull; but on the 
western face they have been cut sharp in huge vertical cliffs at the 
head of deep ravines. The contrast of the two slopes is very strik- 
ing; it can be seen to advantage only from the crest of the range 
itself, north of the teeth, where both their sides are visible, as in figure 
PIE 
The history of the Swazy mass may be more briefly sketched. The 
eastward dip of the strata in this part of the range is but little greater 
than the eastward dip of the block itself. Before the faulting oc- 
curred, the strata must have been nearly horizontal. In a district of 
truly horizontal strata, the result of prolonged erosion would be the 
production of irregular and relatively systemless escarpments. In a 
district of gently dipping strata extensive erosion might produce some 
general alignments, but the alignments would have a considerable 
