42 THE BOOK CLIFFS COAL FIELD. 
AMOUNT OF AVAILABLE COAL. 
Because coal outcrops in the region of which the Book Cliffs field 
forms a part have been traced for several hundred miles along the 
southern, eastern, and northern margins of the Uinta Basin shown in 
figure 1, it is probable that the entire area, consisting of about 14,000 
square miles, is underlain by coal. It has been estimated, however, 
that in the center of the basin the Eocene rocks range from 4,000 to 
6,500 feet in thickness, and as the coal is overlain by this great mass 
of barren strata and also by 1,000 feet or more of the Mesaverde for- 
mation, the total covering of the coal in a large part of the basin is 
deeper than the workable limit. This, at present, under the most 
favorable conditions, is about 4,000 feet. The greater part, therefore, 
can not be considered available, and development is restricted to the 
vicinity of the outcrop. 
A close approximation to the total amount of coal available in the 
Book Cliffs field can not now be made because of the present limit of 
knowledge. A rough estimate, however, can be arrived at by assum- 
ing that the workable limit of the field is 6 miles back from the out- 
crop and that the total amount of coal may be represented by a single 
bed 7 feet thick extending throughout the field. On this assumption, 
there are in the area considered in the present report 1,010 square 
miles of coal land, 360 of which are in Colorado and 650 in Utah. 
Since a cubic foot of coal of 1.3 specific gravity weighs 81.25 pounds, 
and a square mile of such coal 1 foot thick contains 1,132, 560 short 
tons, on the above assumption the total amount is more than 8,000 
million tons. These figures have little value beyond forcibly express- 
ing the fact that there is here an immense coal reserve. 
CHARACTER AND USE OF BOOK CLIFFS COAL. 
PHYSICAL PROPERTIES. 
Though differing somewhat from place to place, the coals of the 
Book Cliffs field possess a rather uniform appearance. They are full 
black, with a slightly brownish streak, and are for the most part of a 
dull tone which commonly is varied by the presence of narrow lustrous 
bands. The coal is traversed by many joints, two sets at right angles 
being prominently developed (PI. IX, B). Accordingly, the coal 
breaks easily after it is mined, but in some of the Avorkings the coal is 
dense and hard to pick. The lumps, as mined, usually range in size 
from 30 inches down, and there is a lar^e amount of slack. Patches 
of a yellow resinous substance occur locally in the coal, and also infre- 
quent small segregations of pyrite. A common feature is the presence 
in the cracks that traverse the coal of thin films of a whitish saline 
