26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
tant peculiarity of this type of mountain front in the present connec- 
tion is that it has been determined essentially by a single deformation, | 
upon which erosion has worked simply to carve the deformed mass, | 
thus leaving its more resistant parts in prominent relief and causing 
the base line to follow the boundary between the strong and the 
weak structures. Such an example may therefore be taken as the 
type of a monogenetic mountain front. It is quite possible that more | 
than one cycle of erosion is here concerned, but the uplift by which 
the present cycle was introduced seems to have been so widespread 
and uniform that it did not significantly affect the structural features — 
of the range. The high-standing crystalline mass of the Front range — 
is generally sharply dissected, yet in certain districts its uplands re- 
tain an accordant altitude which strongly suggests the work of an 
earlier cycle than that now current. This has been pointed out for 
the Front range north of Pikes peak by Emmons. The upland is | 
well seen northeast of South park from the line of the Colorado 
Midland railroad; somewhat farther south, Pikes peak has every 
appearance of being a huge monadnock, rising above the upland — 
level. The valleys that have been eroded in the upland are narrow | 
as a rule, and remain narrow until they pass the foot-hill ridge or — 
hog-back of upturned Mesozoic strata; then the country opens so — 
broadly that the valley is almost forgotten, and it is not at first easy — 
to recognize the great erosion that the Plains have suffered. One of — 
the best points to correct the false impression that the Plains are | 
but slightly modified from their original surface is near Trinidad, Col- | 
orado, where Raton mesa, a mass of weak Plains strata capped with 
a heavy lava sheet, surmounts by some thousand feet the denuded, 
nearly peneplained surface to the north. Certain parts of the Hima- 
layas in northern India offer examples of this type of mountain front, 
but probably less modified by erosion than in the case of the Colorado 
Front range, as may be seen on the road that ascends from the plain 
of the Punjab to Simla. 
A very different type of mountain border is found in polygenetic 
ranges. The type of this class may be taken from among those 
ranges which enclose the intermont basins of Montana, as illus- 
trated, for example, in the Three Forks folio, U. S. Geological 
Survey. Here the mountains are the result of at least two periods 
of deformation, and the second deformation or warping was discord- 
ant with the first; that is, instead of merely intensifying the folds or 
faults of the earlier period, the later deformations introduced a new 
system of warpings. As a result the belts of the earlier deformed 
