ORIGIN OF.CERTAIN FEATURES OF COAL BASINS. 653 
time this area would continue to sink. This action would be 
very slow and probably long continued. As a rule the strata 
covering coal seams are shales and are indicative of slow and 
even deposition. If then a coal basin be formed and coal be 
deposited in it, that coal in the process of solidification would, 
provided the general subsidence of the region continued, thus 
give rise to conditions favorable to formation of a second seam 
above it. This second or upper seam would repeat to a greater 
or less extent the irregularities of the first. The inequalities in 
the thickness of the upper seam would probably not be so great 
as in the lower seam. 
From the above it follows that if in working a coal seam — 
where the general conditions are the same as in the Iowa field — 
a marked basin character is found, it is indicative probably of 
one of two things. Either the coal lies above an unconformity 
and the irregular character of the coal is the expression of that 
fact, or it may lie above another coal seam and its irregular dis- 
tribution may have been conditioned by the equal settling of the 
lower bed. 
How far this may aid in prospecting for coal seams cannot 
now be said. It must not be forgotten in applying it that 
there are many beds other than coal which undergo important 
secondary modifications in thickness. The influence of under- 
ground waters in washing out underlying strata and thus produc- 
ing irregularities in the position of overlying beds may also per- 
haps need to be taken into account. In at least one instance, 
however, there is confirmation of the reality of the process 
sketched above. 
In the region around Des Moines there are a number of 
mines which have at different times worked two seams of coal 
separated by thirty-five to forty feet of shale. The maps of 
these mines have usually been very imperfectly kept and levels 
are only occasionally available. In the Eureka mine the second 
seam is now being worked. The first is a thin seam which has 
no value. The third lies about forty feet below the second and 
has been worked out. In both the second and third seams the 
