throughout the Appalachians, one reason why the mining of anthracite requires engineering skill far in advance of that necessary in the bituminous regions, becomes at once apparent.

The anthracite coal measures consist of an alternation of hard, massive sandstones and conglomerates, with softer black and bluish slates and shales, more or less evenly distributed. It is not possible to base any sub-division of these rocks upon the predominance of hard sandstones and conglomerates at certain horizons, for these may occur at almost any horizon; but we may state, in general terms, that the upper part of the coal measures usually consists of softer rocks than the lower sub-divisions. The hardest rocks are, of course, found in the Conglomerate at the base of the coal measures proper.

Comparing these rocks with rocks of the same age in the bituminous coal areas, we find that they are very much harder, and more silicious, often presenting a somewhat metamorphic appearance.

This is one of the reasons why the rope drilling method of the oil regions has not more rapidly displaced the diamond drill in boring deep vertical prospecting holes.

Structural peculiarities of individual seams. Anthracite, like bituminous coal beds, are subject to the occurrence of irregularities in the roof or floor cutting out a portion of the bed, called by the miner "horse-backs," "horses," or "hog-backs" ;* to a contraction of the seam by close approach of the roof and floor, a "pinch" or "squeeze;" or a bona fide thinning away at the bed; and to "faults "described by the miner as "rock faults," "slate faults," etc., which are not in reality true faults, as that term is understood by geologists, but a partial or entire replacement of the coal by sandstone, slate, conglomerate or fire-clay.

A fault is often called by the miner a "thrust" or

* See Glossary.

STRUCTURAL PECULIARITIES.

AC. 11

"throw," thus "down-throw," " up-throw," etc. They are extremely uncommon in the bituminous coal area of Pennsylvania; I know of but two; one near Brady's Bend in Armstrong county, possibly the result of a landslidethe other is described in the report on Clarion county VV.

They are not uncommon in the anthracite regions, but the amount of movement is generally quite small and the dislocation seems to be confined to the measures immediately inclosing the bed in which the fracture occurs.

These irregularities are shown better than they can be described by the illustrations now being prepared for the geological reports.

When a coal seam is cut sharply off by a fault it is of great importance to determine whether the workings are on the up-throw or down-throw side of the fracture. It may happen that the coal presents a smooth, clean cut face at the line of fracture, giving no indication whatever of the probable position of the other half of the bed, whether above or below, but the line of fracture usually displays some indication of the probable direction of movement. In the vicinity of the fault the coal may be sharply twisted downwards or upwards, or a "tailing" of fine coal mixed with broken rock may be found occupying the fissure. When these indications fail, recourse is had to geological cross sections--and these are only valuable if key-rocks can be indentified on opposite sides of the break-or to bore holes or prospecting tunnels.

Areas of crushed coal are not as common as we might a priori expect from the contorted condition of the coal measures. Crushed coal is commonly found along the crest and in the trough of sharp flexures, but occurs also in many other situations. The loss in mining from this cause is often very great, much of the coal going direct to the dirt dump. When the manufacture of artificial fuel or the consumption of dirt under boilers constructed for that purpose creates a demand for fine coal, the percentage of waste from this cause will be greatly reduced.

Dirt fault. This name is applied to an area of crushed coal, or to a partial or total replacement of the coal by a soft

carbonaceous shale or slate with more or less coal running through the mass in thin stringers. One variety, character ized by greater breadth of the foreign mass at the roof, is doubtless allied to the filled stream channel, described by Mr. John F. Blandy before the American Institute of Mining Engineers.*

Slate fault. A partial and local replacement by slate is termed a slate fault; a simple thickening of a regular slate parting is also often designated by the same term or is called a "horse."

Rock-fault. When the replacing material is sandstone or conglomerate this term is used. Vertical and steeplyinclined masses of rock having nearly parallel faces are sometimes encountered. Their origin has not yet been satisfactorily explained, but in many cases they have doubtless been formed by the filling up of fissures. This term is also improperly applied to a pinch or squeeze when the abutting walls (roof and floor) are sandstone or conglomerate.

The figures obtained from more than one hundred measurements of the different coal beds taken at random, show that the slate, bony, and sulphur partings included between the top and bottom benches, constitute on an average from one fifth to one seventh of the total thickness of the coal. These partings divide the beds into benches varying from a few inches to several feet in thickness, but it is uncommon to find any single bench exceeding eight or ten feet in thickness. While the existence of these partings is usually det rimental, it sometimes happens that a thick parting dividing a large bed into two benches that may be mined as separate beds, is a great advantage, enabling the operators to recover (mine) a much larger percentage of the coal in the ground than could otherwise be obtained. When the top bench is impure and unfit for working a thick solid parting separating it from the lower benches may form an excel. lent roof, and greatly decrease the cost of mining the lower benches.

The irregular distribution of these slaty and bony part

* Transactions, Vol. IV, p. 113.