48 w. Ss. GRESLEY 
CLAY-VEINS INTERSECTING COAL MEASURES. 
So far as the writer knows, these slack-veins are as liable to occur in 
one place as another in the Pittsburg coal field and at any depth; also 
that they pay no regard to the dip, swamps, clay-vein courses, cleavage 
of coal, or joints in the rocks, nor to the existing topography. It seems 
quite clear that the phenomena of the slack or soot veins in the fault 
planes, accompanied by the excessive rolling and folding of the clayey 
and shaly layers, suggest a slow or gradual faulting and not a sudden 
and violent break-up; and also that these faults being in a flat coal 
region are a product of lateral pressure. 
A noticeable change in the dip of the coal takes place on approaching 
a slack vein. 
‘““ HORSE-BACKS”’ IN THE CoAL MEASURES OF WESTERN PENNSYLVANIA 
Another kind of fault in the coal consists of local “rolls” or banks of 
hard, mottled clay, sandstone, and other material, which occupy either 
(1) the horizon of the lower layers of the seam or (2) areas where 
pockety masses of shale, etcetera, have replaced the roof coals and shale. 
It is claimed by some that clay-veins sometimes proceed from or have 
had their origin in these “ horse-backs,” as they are called. The two 
things are often in contact, and cases are known where clay-veins and 
horse-backs are so mixed as to be indistinguishable. 
AGE AND ORIGIN OF THE CLAY-VEINS 
A brief summary of the foregoing facts may be given before proceed- 
ing to discuss the subject theoretically. It has been pointed out— 
(1) That these veins are filled, crooked and ragged, gaping fissures or 
rents in the Coal Measures. 
(2) That the composition of the vein stuff is a compact and irregular 
and varying mixture of various sized (mostly quite small) rough bits of 
shale, limestone, sandstone, coal, etcetera, which cannot be distinguished 
from the composition strata of like materials, whose broken edges form 
the walls of the veins. 
(3) That lateral pressure exerted by the vein-walls on the enclosed 
debris of rocks has been sufficient to produce considerable change in the 
form of the fragments, amounting in places to a structure not unlike 
fluxion, compelling the material to occupy less space than it originally 
did, creating shearing, faulting, and slickensiding in it, and the thrust- 
ing of the stuff into the joints and lamine of the coal in contact with it, 
and producing various amounts of displacement and twisting of the beds 
on opposite sides of the vein. 
