ORIGIN OF CERTAIN FEATURES OF COAL BASINS. 651 
assumed to have been equal at both points. This assumption is 
especially safe since the two points are but a short distance apart. 
As the matter yielded to this pressure, it decreased in bulk until 
in the end it assumed the thickness now seen. The fact that the 
character of the coal at both points is similar confirms the 
assumption that the conditions and pressure at both points have 
been the same. It thus becomes evident that the original dif- 
ference of sixty feet in the thickness of the vegetal matter ulti- 
mately gives a difference of four feet in the thickness of the coal; 
or that each foot of coal now present occupies one-fifteenth the 
vertical space which it originally filled. This ratio has been 
calculated for a number of cases in Iowa and is found in a major 
portion of instances to lie between one to ten and one to fifteen. 
Thus the rule is reached that in the process of consolidation Iowa 
coal seems to have been reduced to from one-tenth to one-fifteenth 
its original bulk. It should be remembered in comparing these 
estimates with those made in other fields that the Iowa coals 
have not been subjected to the action of mountain-making forces. 
There has been no lateral pressure, and the vertical pressure has 
probably been comparatively slight. 
The changes due to this compression would be slight in the 
beds under the coal. To the degree in which they were soft and 
unconsolidated they would themselves be amenable to the same 
pressure. Asa matter of fact they were only slightly affected. 
The fire clay immediately under the coal, if thick, frequently 
shows the greatest changes. It was of course subject to irregu- 
larities of deposition and hence varies in thickness. When the 
strata consolidated the fire clay was more unyielding than the 
coal so that a greater thickness of fire clay is marked by a 
‘‘horseback”’ or ridge pushed into the coal. Changes in the 
thickness of the coal bed would not greatly affect the under- 
lying strata except to the slight degree in which they may at 
times have lessened the pressure on the fire clay. The effect of 
such a decrease would correspond to that now seen in “ creeps”’ 
which are due to a similar cause. 
In the coal bed itself the effects of the settling may be seen 
