Geology of Casile Rock.—Lee. 109 
_ of the northern limb. When the change in folding which had 
formerly been by north-south thrust, was changed to an east- 
west thrust, the tilted syncline was doubled on its axis. At the 
base were hard strata—the sandstones and conglomerates of 
the Red Beds, the Dakota sandstone and the Colorado lime- 
stones. The body of the trough was filled with thousands of 
feet of soft shale. When the folding occurred, there was a 
tendency to bring the lower hard strata which had been pre- 
viously bent downward in the form of an arc, into a straight 
line, thus shortening the line of outcrop. This shortening was 
accompanied by a breaking and arching of the harder strata 
into the soft shales above. The attitude of these sections may 
be illustrated by bending pieces of cardboard first into the 
form of a trough, and then doubling them upon their axis. 
As a corollary of the foregoing discussion, the question 
arises whether the great thickness of the shale in Perry park 
and elsewhere is due wholly to deposition, or due in some meas- 
ure to mechanical thickening produced by the movements 
which upturned the strata. It is only in the disturbed regions 
along the mountain front that great thicknesses such as that 
found in the Castle Rock and Denver regions occur. Where 
the shales lie in a horizontal position in undisturbed regions 
they do not, so far as known, attain such thicknesses as those 
shown in the upturned belt along the mountain front. A short 
distance, a mile or two at most, the various formations are 
found in a nearly horizontal position. It follows from this 
that the angle of bending lies at no great depth and that the 
shales, lying as they do above the hard strata, would be strongly 
compressed in the angle of flexure. Soft shale, such as that of 
the Ft. Pierre, would probably act as a plastic body in case of 
disturbance. This seems to be exemplified in Perry park. The 
more resistent layers, the Dakota sandstone and the Colorado 
limestone, broke and moved in blocks as previously explained. 
No evidence of such breaking appears in the shales. It is 
probable that while the original thickness of the shales in this 
region was considerable, it has been very materially increased 
by mechanical thickening. If this be true, the estimates of the 
amount of elevation and depression made from the observed 
thickness of the shales should be materially modified. 
Trinidad, Colo. 
