100 The American Geologist. February, 1902. 
The buttes of Perry park, if they can properly be called such, 
are more irregular in form and composition than those from 
the undisturbed region from which they were originally de- 
scribed, and the shells, although numerous, do not form so 
large a part of the mass. The species found in greatest abund- 
ance is Lucina occidentalis. 
12,—Arapahoe,—The type area of the Arapahoe is the 
Denver basin, lying immediately north of the Castle Rock area, 
This formation together with the Denver beds, lies above the 
undoubted Laramie and below the undoubted Tertiary. The 
Arapahoe is separated from the Laramie, as shown by Mr. 
Cross,* by a notable time division. But because of the dino- 
saurs found in it (Ceratops), the Arapahoe has been referred 
to the Mesozoic. The geographical extent of this formation is 
unknown. Mr. Emmons f states that “vertebrate fossils char- 
acteristic of the post-Laramie formations have been observed 
by professor Marsh in Monument park (a few miles south of 
the limit of the Castle Rock region) and remnants of beds re- 
sembling the Arapahoe and Denver have been observed near 
Canon City which may have been contemporaneously depos- 
ited, but whether the lake was continuous along the mountain 
front, or there were several small isolated basins, it is as yet 
impossible to determine.” In the sarme connection he states 
that at the base, occurs 50 to 200 feet of conglomerate which 
contains fragments of nearly all the older formations of the 
region. This conglomerate is found in the Cas le Rock region. 
It was observed near the northern border where it extends 
southward from its type area, the Denver basin. Thence south- 
ward, it was observed at short intervals to a point southeast of 
Perry park, where it disappears beneath the Monument Creek 
beds. It reappears again about six miles south of the southern 
border of the region mapped, and extends thence beyond the 
area examined. In the Denver basin the conglomerate is over- 
lain by a shale series. No shales were observed in t e Castle 
Rock region which could be definitely referred to the 
Arapahoe, 
The: conglomerate stands nearly vertical and forms a line 
of prominent monuments. In composit’on it resembles closely 
*U. S. Geol. Surv., Mon, 27, p. 207. 
*S. F. Emmons, U. S. Geol. Surv., Mon, 27, p. 31. 
