ORIGIN OF CERTAIN FEATURES OF COAL BASINS. 647 
intermediate regions. The exceptions to this rule are rare and 
are usually found either where the rise is slight and covers a 
small area or where the coal is of an impure or bony character. 
In all the greater differences the coal thins towards the rise. 
These lower lying portions of the coal beds are usually called 
by the miners ‘‘swamp coal” or ‘basin coal”’ and it is in these 
swamps or basins that the thicker and more productive portion 
of the beds he. In many cases these basins have been traced 
considerable distances, maintaining the general character of a 
trough with coal thick at the center and thinning on each side. 
The area they cover is usually quite irregular. Ina few cases 
the bed underlies a broad circular area thinning quite regularly 
as it rises in all directions from the center. More usually the 
length in one direction is much the greater, instances being 
known where areas of workable coal only about 100 yards wide 
have been traced nearly a mile. They do not run in direct lines 
but curve often in an intricate and irregular manner. They are 
not confined to any one locality but are found in such widely 
scattered regions as Scott, Keokuk, Jasper, Marion and Lucas 
counties. Indeed they are present in nearly every region in the 
state in which coal is mined and are largely sought for by 
_ operators. They seem to be more abundant and more irregular 
in character in the lower horizons. The general character of 
such a coal basin is represented in Fig. I, a composite section. 
On the one side the coal rises twenty feet in a distance of 320, 
thinning from six to four feet. On the other hand it rises sixty 
feet in 400 and thins from six to two feet. These two instances 
are taken from a large number which might be given. The rise 
of sixty feet is rather unusual, the variation being more frequently 
from twenty to forty feet. In mining, neither the total variation 
in elevation nor in the thickness of the seam is usually learned, 
since so soon as the coal becomes too thin for profitable mining 
it is rarely traced farther. 
A number of these cases have been examined and the rela- 
tions between the amount of rise and the decrease in thickness 
determined. The cases examined yield a series of ratios varying 
