50 Ww. S. GRESLEY—CLAY-VEINS INTERSECTING COAL MEASURES. 
traversing the denuded Pittsburg coal—are questions of considerable 
geologic interest. 
The questions a and } cannot be answered until the lower coals are 
mined vertically below the upper ones, and the clay-veins in both or all 
of the seams shall have been surveyed and mapped. 
The Sewickley or Redstone coal, lying 60 to 80 feet above the Pitts- 
burg bed, is very commonly reported to be much more seriously 
clay-veined than the latter; but the writer has failed to find any oper- 
ator in it who keeps a plan of his mine, not to mention one of his clay- 
veins. 
In the absence of proof to the contrary, it cannot be denied that dur- 
ing accumulation the Coal Measures may have been fissured and clay- 
veined more than once; and if so the question of their ages may event- 
ually be determined by extended inspection and study of them. Could. 
the period to which the fossil plant remains (see figure 15) belong be 
~ fixed, then something definite would be obtained. Possibly some day a 
flint arrowhead, a bone, or a shell may turn upin a clay-vein. Remem- 
bering that in various parts of the world fossiliferous fissures occur, 
whose ages (determined by the fossils) vary from the Permian to the 
Recent, it is evident a great range of time is allowed in which the western 
Appalachian and Interior coal measures may have been veined in this 
way. 
As to how and why these gaping fissures came into existence the fol- 
lowing suggestions are offered: The ragged and crooked forms of the 
veins leave no room for doubt that the strata were pulled apart laterally 
in such a way as to tear the rocks vertically in almost every direction 
(see figures 6 and 7, page 40) ; moreover, as the clay-veins exhibit no signs 
of having been formed at different periods, the inference is that such 
splitting open of the strata occurred at about one and the same time. It 
is possible that the fissuring was done suddenly, as fissures are produced 
by earthquakes today, but their reticulated appearance suggests a slower 
process of formation. Could shrinkage of the earth’s crust have caused 
this peculiar breaking up of the strata into rude fissure-separated masses, 
somewhat after the manner of septarian formation, mud cracks, etcetera ? 
Surface cracks in plaster, asphalt, concrete, and similar material often 
remind one, more or less, of plans of the clay-veins. 
Irregular crustal elevation, accompanied by some torsional movement, 
might, perhaps, better explain the facts; or would a series of shifting 
or wave-like subsidences or depressions, on the sloping sides of which 
such fissuring might take place, suit the phenomena better? 
Probably the character of the vein material implies that the tops of 
the fissures were above water when the strata opened. At any rate, no 
a 
