} 
108 The American Geologist. February, 1902. 
were deposited. 2, A local depression took place at the close 
of the Red Beds period making conditions favorable for the 
deposition of gypsum. The uneven base of the gypsum sug- 
gests that the floor upon which it was deposited was an eroded 
surface. If this be true, it natura ly follows from the absence 
of the gypsum on both sides of Perry park, either that the re- 
gion in general was above sea level during the gypsum form- 
ing stage, or that the gypsum which may have been deposited 
beyond the limits of the park, was removed by subsequent 
erosion. In the first case its deposition may be explained by 
local depression; in the second case its preservation may be 
due to such depression. 3. The presence of the Dakota and 
the Colorado formations in the center, and their disappearance 
at the extremities of the park, indicate either that they were 
formed in a depression between two land surfaces, or that 
their extremities were carried away by erosion. As already — 
stated, the former is the more probable. 4. A still further de- 
| pression occurred during the Colorado period, allowing the 
Lams 
| 
Niobrara limestones to extend beyond the Dakota. The Col- 
orado hog-back composed principally of the limestones of the 
Niobrara epoch, assumes a prominence in Perry park such as 
is found nowhere else in the region. In general, along the 
mountain front of central Colorado, the hog-back east of the 
Dakota is of small importance and formed principally of a 
hard stratum of Niobrara limestone. Within the limits of the 
park this hog-back attains a prominence equalling that of the 
Dakota and is composed of Niobrara limestones the lower 
stratum of which has already been referred to as different 
from anything found in that formation elsewhere in the vicin- 
ity. 5. At the close of the Niobrara epoch a notable depres- 
sion of the syncline occurred, allowing a great accumulation 
of shale. This depression affected a wider area than other 
movements had affected, as shown by the extension to the 
south of the thick shale formation. Owing, however, to the 
covering of the shales by the Monument Creek beds, the south- 
ern extent of this depression is unknown. 6. No other move- 
ment is indicated until the close of the Laramie epoch. This 
movement seems to have affected the syncline in a peculiar 
way. I have already called attention to the probability that 
the Perry park block was tilted to the north by a depression 
