466 PROF. H. D. ROGERS ON THE LAWS OF STRUCTURE 
Cleavage in Red Shale caused by a Trap Dyke parallel to its walls, near New Hope, Pennsylvania.* 
Fig. 8. 
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Cleavage superinduced by a Trap Dyke in Red Argillaceous Sandstone of Jurassic age, west of Gettysburg, Pennsylvania. 
Other instances are often presented of masses of superincumbent trap rock, baking 
and altering argillaceous and other strata, in which a like law of parallelism of 
the cleavage to the heat-imparting surface of the molten matter is shown in the 
horizontality of the cleavage planes, whatever be the dip of the strata. Numerous 
examples can be cited, where one igneous dyke cutting another, or traversing a 
mass of earlier Plutonic rock, produces in the latter a crystalline grain, amounting 
to a sort of cleavage, adjoining the bounding surfaces of the newer injection, and 
in planes invariably parallel, or nearly so, to the walls of the fissure. A similar fact 
of the occurrence of a cleavage parallel to the walls of highly heated fissures, may 
be seen in the faults and great dislocations which traverse some parts of the anthra- 
cite coal basins of Pennsylvania. Here the greatly indurated argillaceous shales, 
and even sometimes the coal itself, display a cleavage-structure invariably parallel 
to the general plane of the fracture. Such fissures would be the natural channels 
through which heated volcanic steam would ascend from the interior, and the 
action of this upon the strata most susceptible of cleavage would be precisely 
analogous to that of a molten dyke, in transmitting a wave of heat perpendicular 
to its surface, partially softening and half polarizing the matter as it passed. 
* There is a similar instance cited by Professor Purxurs, I think in his Geology of Yorkshire. 
