ORIGIN OF CERTAIN FEATURES OF COAL BASINS. 649 
limestone ledges of the two opposite sides of the basin may be 
as directly correlated as those on opposite sides of a modern 
stream. In working the coal in this mine, which is merely a 
small country bank, it has been found that the coal dips toward 
the present river. Thus it seems that this vein was laid down 
in a small gully or ravine opening out into some larger basin, the 
margin of which was located approximately along the present 
course of the Des Moines. 
In these and other cases it is at once seen that the lower line 
FIG. 2. 
is the cross section of a previously existing ravine. There is no 
reason to doubt that a similar statement would hold in a great 
number of the remaining cases. In all places where there has 
been an opportunity to examine the underlying rocks this has 
been found to be true. Ina majority of the other cases known 
the coal horizon lies undoubtedly above an unconformity of ero- 
sion, and the underlying rocks are known to have suffered ero- 
sion sufficient to account for just such outlines. Not all, how- 
ever, are known to be so located. In a few instances similar 
basins are found in seams which so far as known do not overly 
any unconformity. In the veins which lie high up in the coal 
measures the coal often has a rolling or undulatory character, 
differences of twenty to twenty-five feet in elevation being not 
uncommon. In such cases there is not always a constant rule 
as to the thinning of the coal over the higher points. It is only 
as the basins are traced out to their limits that the rule appears 
to hold. These minor changes can readily be accounted for by 
the secondary changes induced by the consolidation of the 
