250 
ELDKIDGE. 
General Features of the Affected Area. 
The area affected by the phenomena now to be discussed 
extends along the base of the foot-hills of the Colorado Range 
west of Denver, from a point about a mile south of Bear 
creek northward to Coal creek, a distance of 21 miles, with 
a breadth varying from 2J to 4 miles, the greater occurring 
along its northern and southern edges. It involves the hog¬ 
backs of the Dakota and the region within to the Archaean, 
and includes the prairies as far to the east as Mt. Carbon, 
the western slopes of Green Mountain, the Table Mountains, 
and the vicinity of the Ralston dyke. 
Topography .—Its topography shows a marked variation 
from that normal for the foot-hills region in general, and its 
relations with the geology of the tract as displayed from 
point to point throughout its extent are so close as to warrant 
the assertion that for every topographical lineament there is 
to be discovered an equivalent geological incident that has 
led to its development. The normal foot-hill topography 
consists of a mountain mass of Archsean rocks, fringed at an 
average distance of half or three-quarters of a mile by a sharp 
serrated ridge of Dakota sandstone, the valley between the 
two being occupied by the formations of the Trias and Jura. 
Above the Dakota in their geological succession come the 
Fort Benton, the Niobrara—this generally constituting a 
second, smaller reef outside the Dakota—the Fort Pierre, 
Fox Hills, and the Laramie, the basal sandstones of the 
Laramie, again, forming either a low roll in the ground or 
an actual comb of rock slightly projecting above the surface 
of the surrounding prairie. To the east of the Laramie, at a 
distance of between 600 and 1,200 feet from its basal sand¬ 
stone, appears in the southern portion of the area yet another 
comb, formed by the conglomerates at the base of the Arapa¬ 
hoe series. Finally, this is followed at about an equal dis¬ 
tance by either an outcrop of the lower members of the 
Denver formation or a peculiar ribbing of the prairie due to 
their presence beneath the surface. For mile after mile 
