4 
OCCURRENCE OF COAL WITH UNUSUAL CONDITIONS 329 
WHEN WERE THE DEPOSITS COMPRESSED TO APPROXIMATELY 
THEIR PRESENT THICKNESS ? 
There is yet another feature to be considered, the time when the 
reduction in volume of the coal mud occurred. Evidence has 
already been cited to show that this occurred while the sandstones 
were yet unconsolidated. As has been repeated, the accommoda- 
tion of the surrounding sediments to this shrinkage was, in large 
part, by movement in the underlying shale. ‘There was also some 
readjustment in the lower part of the sandstones, as shown by the 
irregular lenses between the coal stringers. But whether there was 
any very appreciable movement in the main body of the sandstone 
overlying the coal is not known. There must surely have been 
some. In one of the pockets, there are 12 to 14 inches of coal 
along a width of 60 feet, in another, 35 inches of coal are found in 
a rapidly pinching lens. Both of these cap unusually high shale 
crests, but it seems impossible, in these instances, that the thick- 
ness of vegetable mud necessary to form them, at the very least 
t5 feet, could have been compressed to its present state without 
allowing the overlying sandstones to settle slightly and irregularly. 
But the shale bed and coal which follow next above the sandstone 
are evenly horizontal and continuous and show no evidence of any 
such irregularity. This coal was formed as a sheet extending con- 
tinuously over a wide area, and in a manner differing radically in 
detail from the one at the base of the sandstone. Its regularity 
in the cut is such as to suggest that equilibrium had been quite 
fully established in the underlying sediments before it was laid 
down although proof positive to this effect is not at hand. In 
other words, it is probable from the evidence furnished by this 
occurrence that some of the organic deposits which later formed 
the soft coals, perhaps all, were compressed nearly to their present 
volume very soon after accumulation. This loss of volume was 
probably chiefly due to the pressing out of the large quantities of 
water which must have been inclosed in the deposit at the time of 
accumulation. The loss of the volatile gases which marked the 
ultimate change to coal must have been accomplished more slowly, 
although it seems possible that a part of this, too, occurred at the 
time of this first loss. The not infrequent finding of coal pebbles 
