AGE CHARACTERISTICS OF COALS 
147 
Mexico, as well as in the southern tier of countries in Wyoming. 
The Middle Pierre or Mesa Verde succession is probably the 
most productive formation of all, with usually one or more work¬ 
able seams; but its seams are like those of the newer formations. 
They are variable and uncertain in New Mexico. In the Uinta 
Basin, west of the Grand River, portions of the section contain 
workable beds of coal in some districts but are entirely barren in 
others; east of that river the coals are local, important here, un¬ 
important there, or absent elsewhere. Mesa Verde coals of the 
Green River Basin attain commercial importance in only one 
county. In Montana the lenses are usually small and thin. In 
Alberta the coals are present in great area and are often work¬ 
able, but available details merely suggest that they are lenses. 
Coal lenses ordinarily show increase of foreign matter towards 
the borders. The seam becomes broken up by fine partings, and 
very often it passes at last into merely impure carbonaceous shale 
with thin laminae of coal. Sometimes the lenses are connected 
by a stretch of black shale; but commonly no such bond exists, 
and a barren space intervenes. These lenses, great and small, 
are similar to the peat deposits on the broad river plains, and even 
more strikingly resemble those on coastal plains. At times these 
are separated by broad spaces which are forested. At other times 
they are united by carbonaceouse muds; while in still others the 
peat of several lenses has become continuous by transgression. 
In all coal-mining districts the effects of contemporaneous 
erosion are conspicuous. Curious intermingling of coal and 
debris, observed at one locality in the Loewenberg area of Silesia, 
seems to be explicable only on the supposition that it represents 
a washed-out swamp. The presence of coal grains in certain 
sandstones may signify that a coal seam in process of formation 
m 
was exposed. Local conglomerates in many sandstones occupy 
old channel-ways of rapid streams. Local unconformities between 
sandstones and shales suggest changes in direction of drainage. 
Coal seams themselves appear to have been often subjected to 
subaerial erosion, and to have been frequently traversed by 
streams as in modern swamps. “Horsebacks,’’ or rolls, of the 
roof have been found wherever extensive mining operations have 
been carried on. They mark channelways of varying width and 
depth, now filled with materials like that of an overlying deposit. 
Sometimes the material is the same as that forming the immediate 
