Geology of Castle Rock.—Lee. 103 
within the region mapped, but further to the east they are 
water-worn. These upper beds, then, differ in character from 
those of the lower division; they are separated from it by vol- 
canic tuffs and flows of rhyolyte and by an unconformity rep- 
resenting a period of erosion as shown by the presence in it of 
the material from the underlying rhyolyte. It is, therefore, 
separate and distinct from the lower division. Added to this 
is the obvious inconvenience of a cumbersome name. For these 
reasons I have ventured to restrict the use of the name, Monu- 
ment Creek, to the lower division and suggest a new name, 
Castle conglomerate, for the upper division,—the name is de- 
rived from the typical development of the formation on Castle 
Rock butte. 
The name originally applied by Hayden is thus retained 
for the formation which is best known, and a new and less 
cumbersome name ‘given to the less extensive and little known 
formation. 
The areal distribution of the Monument Creek, the rhyo- 
lyte, and the Castle conglomerate is not given on the accom- 
panying map. They lie for the most part east of the region 
shown in the map. The rhyolyte is found on or near the tops 
of the buttes and mesas which abound in this region, and ex- 
tends from the northern border southward to Palmer lake, and 
eastward something like twenty miles from the mountains. 
The Castle conglomerate extends from the northern border 
southward beyond the center of the Castle Rock region, and 
eastward something like fifty miles. 
Structure. 
1. Castle Arch—tThere are two notable structural features 
in this region,—the Castle arch and the Perry Park syncline, 
The arch is a structure similar to the one near Golden, Colo- 
rado, described by Mr. Eldredge in the “Geology of the Den- 
ver Basin.” The Castle arch is found west of Castle Rock— 
the town from which the name is derived. For want of a 
better objective point, I shall refer to the place where the Mon- 
ument Creek beds come in contact with the crystallines of the 
mountains, as the crest of the arch. The relations of the sev- 
eral formations to the arch and to each other are unfortunately 
obscured near the base of the mountains by crystalline debris, 
especially near this crest. Put several points could be made 
