652 WZAS JO WISINAIL, (OME CSE OULIOG WZ 
in the small slips or faults which are encountered. As the coal 
became older and less plastic it was able to resist the pressure 
for a considerable time. Finally the accumulated force would 
cause it to yield, and this yielding would take the form of a 
series of slips or of cracks along its edges rather than of a grad- 
ual unbroken deflection. Since the lower beds were more largely 
resistant the faults do not often extend far into them and fre- 
quently do not even extend to the bottom of the coal. 
In the beds above the coal the effects of this action must be 
large. The shales which lie immediately over a coal seam are 
frequently quite badly faulted. This is usually more evident in 
bituminous than in argillaceous shale. This is probable due to 
the fact that just in proportion to the degree with which they 
are charged with bituminous matter they would themselves undergo 
the same loss and other changes which occurred in the coal. In 
general, however, shale accommodates itself readily to pressure. 
Limestones and sandstones prove more unyielding and break 
with sharp fault lines into the underlying coal beds. 
One of the most important results of these changes and the 
one which gives them their deepest economic significance is the 
part which such a change in a coal seam would play in inducing 
the deposition of overlying seams. Starting with the conception 
of a coal swamp as a low-lying coastal marsh it will be seen that 
any irregularity in the surface which makes an inreaching basin 
will afford better opportunities for the formation of a coal seam 
since it will give a better—-more protected—area for the 
growth of plant life and will shield the deposit from the incur- 
sion of currents. The influence of such preéxisting irregulari- 
ties is believed to be seen in the coal seams already described, 
as for instance that at the Vanderberg mine. Now if there be a 
sufficient cause for the prolonged and gradual subsidence of the 
area, the ideal conditions for the formation of a coal seam would 
be present. It must be evident that in the course of such sec- 
ondary changes in a coal body as has been sketched above there 
would be, unless sedimentation were active, an area over each 
coal basin which would become depressed. For a considerable 
