36 W.S. GRESLEY—CLAY-VEINS INTERSECTING COAL MEASURES. 
The comparative obscurity of clay-veins confines their inspection and 
study i situ almost entirely to such parts of them as are laid bare by 
such mine workings as these veins or dikes intersect or disturb. In other 
words, it is only a very small part, indeed, of a clay-vein that can be seen 
and examined, for a miner will cut or cross as few of them as possible. 
Nevertheless, many of them occur, and are of necessity cut and thus ex- 
posed in one place or another and at one horizon or another in the Coal 
Measures. 
DEFINITION OF A CLAY-VEIN 
A clay-vein (also termed “ clay-seams,” “‘ mud-seams,” etcetera, by the 
miners) may be described in a general way as a more or less vertical, 
crooked, tortuous, often branching, ragged sided wall or dike intersecting 
a seam of coal and composed of (1) compact, indurated, clayey materials, 
-or (2) a mixed debris of rocks, forming a breccia or a conglomeratic mass 
varying in width from a mere streak or film of clay to as much as 15 feet, 
but averaging about 10 inches. 
SOME GENERAL CHARACTERISTICS OF CLaY-VEINS 
The horizontal trend of a clay-vein may be in any direction. It may 
extend a few yards only or run a mile or more before being lost to sight. 
It is liable to branch or divide both vertically and horizontally, and even 
to unite again. Local swellings and sudden nips occur, and it may have 
toughened, twisted, spoiled, and somewhat displaced the coal or other 
material forming its walls. It is frequently joined to or forms part of 
one or more other clay-veins, thus constituting a member or branch of a 
system or rude network of such veins, traversing possibly an entire coal 
field, and being devoid of cavities and extending downward below the 
coal, as well as upward through the roof, to unknown distances. Clay- 
veins have been met at amaximum depth of about 400 feet, but probably 
occur at greater depths. — 
DerratLepD Derscriprion or A TYPICAL CLAY-VEIN 
Figure 1 represents a vertical section of an ordinary clay-vein, as 
sketched by the writer, in a mine working the Pittsburg seam of coal in 
Westmoreland county, Pennsylvania, at a depth below the surface of 
about 300 feet. ‘The leading features of this vein are as follows: ‘The 
clay-vein cuts the coal seam where it is of normal height, quality, dip, 
etcetera. The vein is about 1 foot in width, but splits in the roof coals 
and shales, detached mass of which may be regarded as a “horse” in 
the vein. There is a vertical displacement of the coal bed amounting 
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