106 The American Geologist. February, 1902. 
south side, than on the north side of the crest. . The somewhat 
extensive erosion which took place north of the crest previous 
to the deposition of the Arapahoe and which is shown by the 
truncation of the older formations is perhaps best explained by 
such a postulate. No such fault was located, but this could 
scarcely be expected since the Monument Creek beds extend 
nearly if not quite to the crystallines at this point, thus cover- 
ing any fault lines which may have existed at the surface pre- 
vious to the deposition of these beds. 
I have postulated an east-west arch in the belief that it 
offers the best explanation of the stratigraphical distribution of 
the region. The most serious objection seems to lie in the fact 
that the relations at several critical points could not be defi- 
nitely determined owing to the surface debris. Some of the 
phenomena could be satisfactorily accounted for by the tilting, 
faulting and thrusting which accompanied the mountain form 
ation. But these movements, while they played an important 
part, do not seem adequate to explain many of the relations 
such as the absence of the Dakota, the thickening of the Ft. 
Pierre shales, etc. It should be borne in mind that the sedi- 
mentary formations of this region have been upturned to a 
nearly vertical position. The width of the exposures, there- 
fore, as shown in the accompanying map, nearly represents the 
thickness of the formations. If the map be turned so that the 
west side forms the base, it serves as a vertical section, except 
in the case of the mottled beds of Perry park which lie at a 
low inclination. . 
2. Perry Park Syncline,—(Perry park should properly be 
defined as the space included between the mountains and the 
vertical wall of Red Beds which forms the eastern boundary. 
For convenience in the following discussion, however, I shall 
refer to the whole region containing the upturned Mesozoic 
formations shown in the center of the map, as Perry park.) 
The second structural feature of the region is the Perry park 
syncline. It lies immediately south of the Castle arch and the 
two structures are probably correlative. The southern limb of 
the arch forms the northern limb of the syncline. Many of the 
observations, therefore, made in the discussion of the arch 
apply also to the syncline, and if it were not for data more or 
less distinct from those connected with the arch, the peculiari- 
