fenneman.J THE BOULDER, COLO., OIL FIELD. 331 
beds. The same is true of the Benton, whose shales are character- 
isticalty dark and whose bituminous odor is at least as well marked 
as that of the Niobrara. The Dakota bears oil in Wyoming, and 
asphalt oozes from its cracks at various places from Wyoming to 
southwestern Colorado. Even the Morrison beds contain some oil, as 
seen near the Florence field. 
The strata thus enumerated have a combined thickness of from 
5,000 to G,000 feet below the horizon of the lowest oil reached in the 
Boulder field. Not all parts of this great thickness are equally prob- 
able sources of the oil. The lower beds of the Pierre are usually 
darker in color and richer in organic matter than those horizons 
immediately below the Boulder oil. The best of the "oil springs" 
also point to a source not higher than the principal sandstone stratum, 
which is about 2,000 feet above the base of the Pierre. The Benton 
and Niobrara are probably richer in oil than any higher strata, and 
surely in this immediate locality far richer than anything below. 
The more probable sources, therefore, lie between the top of the 
Dakota and the middle of the Pierre, a thickness of strata probably 
limited to 4,000 feet. 
Of these 4,000 feet probably containing oil, from 400 to GOO feet of 
the most bituminous beds (the Benton shales) lie below the compact 
basal limestone of the Niobrara. This limestone, though less than 30 
feet thick wherever quarried for lime west of the oil territory, is very 
dense, and where unbroken must probably prohibit the accumulation 
of oil above from sources below this horizon. That it is unbroken 
beneath the oil field is by no means certain. Within 5 miles (the 
vicinity of Marshall) faults abound, many of them having displace- 
ments far greater than the thickness of the Niobrara basal limestone. 
Pronounced rupturing of the strata is shown within 2 miles, by the 
Valmont dike. Such faulting as that at Marshall could not be 
detected on the outcrop of the homogeneous and easily weathered 
Pierre shales. 
The folding which is almost certainly present would easily joint or 
brecciate the brittle Niobrara limestone to such an extent as to make 
it no barrier to the accumulation of oil above from the carbonaceous 
constituents of the beds below. For the present, therefore, the 
Benton shales should be taken into account, along with the higher 
strata, in the consideration of possible sources of oil in this locality. 
PRODUCTION. 
Since January 1, 1901, there have been built within 5 miles of the 
McKenzie well about 120 derricks. At 82 of these, wells have been 
drilled to depths varying from 200 or 300 to 3,400 feet. In addi- 
tion to these, 13 scattered wells have been drilled at various distances 
from the foothills, both north and south of Boulder from the Cache 
la Poudre River on the north to Coal Creek on flic south. Of these 
