Geology of Castle Rock.—Lee. 107 
ties of the region might possibly be accounted for without post- 
ulating a syncline. Some of the more conspicuous of these 
peculiarities are as follows: 1. The presence of Carbonifer- 
ous strata in Perry park while nothing definitely referable to 
that age is known for a considerable distance either north or 
south of the park. 2. At the top of the Red Beds, a heavy 
stratum of gypsum occurs which 1s confined to the park. It 
thins out in either direction. Its thickness varies abruptly, due 
in some cases at least, to irregularities in the floor upon which 
it was deposited. 3. The Dakota sandstone is present in nor- 
mal development but thins out at either extremity of the park. 
4. The Colorado formations make a prominent hog-back with- 
in the park, but thin out toward the extremities. Some mem- 
bers of the Colorado group, however, extend for some distance 
beyond the points where the hog-back ceases. There is one 
limestone layer about four feet thick which seems to be pecu- 
liar to the park. It was not found elsewhere in the region. It 
is very hard and composed principally of fragmentary shells. 
It forms the crest and is the main cause of the prominence of 
the Colorado hog-back within the park. 5. The local thick- 
ening of the Ft. Pierre shales has been referred to, perhaps 
sufficiently. It should be noted in this connection, however, 
that there is evidence of thinning toward the south, although 
it is not so conspicuous as in the northern limb. 6. A some- 
what different group of data is found in the attitude of the 
hog-backs in the park. The upturned strata composing them 
are broken into four distinct sections. Unfortunately these 
sections which are so distinct in the field cannot be adequately 
represented on the topographic map from which the accom- 
panying map is taken. The first section to the north strikes 
N. 7°E. with the strata either vertical or overturned to a 
greater or less extent. The second section strikes N. 57°W. 
with nearly vertical strata. The third strikes practically east 
and west with strata dipping less than 10°. The fourth, 
southernmost turns again to a nearly north-south hedieies 
with the strata vertical. 
These peculiarities both of form and structure, seem to be 
best accounted for by postulating a synclinal structure of long 
duration. According to this postulate there was: 1, a synclinal 
trough in the crystallines in which the Carboniferous strata 
