rock. The No. 2 bed is 22 to 30 inches thick, averaging about 27 inches. The No. 3 bed averages a little thicker, about 30 inches.

If the beds were mined separately, there would naturally be a substantial loss of coal from one bed or the other. To mine one bed almost completely would be quite possible but would probably entail heavy loss in mining the other, and it would make little difference which bed was mined first. If both beds could be mined simultaneously, a high percentage of recovery from each might be possible; therefore the company decided to try the experiment. Ordinary methods of handling the intervening rock would have made the cost of production prohibitive, so an entirely original method of mining was evolved.

The mining methods used are described, but a preliminary description of the measures in this area should be helpful. Table 13 gives a section of the bed at a point about 2,000 feet northeast of the opening at the place marked A in Figure 5. Section B was measured 250 feet due east of section A. (See also fig. 5.)

Table 13.—Sections of No. 3 Dunmore bed in Gray slope area, Forest City

colliery

The log of borehole 2,452, near these two sections, gives the following succession of strata from the surface to the bed of coal:

Table 14.—Log of boreholes at Forest City colliery

These boreholes indicate that the cover over the bed is about 106 to 242 feet thick.

Mining methods.—The method of opening the bed is similar to that for any other fairly flat bed of coal. Cross headings are turned ap

so that water will be drained off. They are driven 12 feet wide, with an airway of the same width. Chain pillars at least 25 feet thick are left between the heading and the airway. All rock from the divider in the heading and airway is loaded into mine cars and removed to the outside, except in a few places some of this rock could be gobbed by removing part of the coal from the lower bed in the chain pillar. Rock has also been hand packed into the crosscuts between the main gangways and the airways.

Rooms are turned to the rise on 50-foot centers and are driven to within 30 feet of an airway of the heading above. These rooms are 10 feet wide and are cut through the No. 2 Dunmore bed, the parting rock, and the No. 3 Dunmore bed; then enough coal is removed from the bottom bed of the No. 3 Dunmore to provide space for the rock that is shot down from the divider. In advance mining the coal is taken from the left rib of the No. 3 Dunmore, but is left in place on

the right rib, so that the gob will not interfere with the recovery of any coal left in the pillars in the No. 3 Dunmore. No coal is taken from the top bed in advancing except the 10 feet mentioned, which provides headroom for the mine cars.

In the locality represented by section A the divider was so thick that all of the coal had to be removed from the bottom of No. 3 Dunmore to provide space in which to pack the rock. Figure 5 shows that in the section marked " C " no pillars were left in the No. 3 Dunmore bed while advancing, the entire side being supported by rock hand packed in the space from which the coal had been removed.

In the locality marked "/?," from which the second section is taken, the reader will note that the parting was not quite so thick; consequently, there was no need of removing all of the coal from the bottom bed and some pillars were left, a condition also shown in the top half of Figure 6. When a chamber has been driven its full dis

mined in the retreat, as are the few pillars that are left in the No. 3 Dunmore bed.

When all or nearly all of the coal in the No. 3 Dunmore bed is removed in the advance, the dividing rock tends to settle on the pillars; when the No. 2 Dunmore bed is mined it separates to some extent from the dividing rock and is worked very easily. In some places the system almost becomes a semilongwall in advancing, as all of the coal is taken in the bottom bed; the same thing is true of the top bed, the mining of which becomes semilongwall in re

treating. Figure 7 shows the relation of the workings in both beds. The solid lines show the workings in the No. 2 Dunmore bed and the dotted lines the workings in the No. 3 Dunmore. Figure 8 shows the workings in the No. 2 Dunmore.

Operating data.—An inspection report of the Hillside Coal & Iron Co. for 1916 offers the following data: In No. 2 and No. 3 north headings, 32 miners, 15 miners' laborers, 1 motorman, and 1 brakeman pro

In the No. 1 north heading, 26 miners, 10 laborers, 1 motorman, and 1 brakeman produced an average of 50 cars of coal and 10 cars of rock daily for one month. No undercutting machines were used, and the coal was blown off the solid; the only system followed was to blow the coal' toward the roadway.

FIGURE 8.—Map of No. 2 Dunmore bed. Black areas are second mined

Bed mined. In the district near the extreme eastern part of the property in the No. 2 shaft, which is not shown on the maps, a similar system was used, but some difficulties were found that will be of interest.

Table 15.—Section of bed in No. 2 shaft district, Forest City colliery

The cover at this point was about 480 feet thick.

Mining methods.—The lower half of Figure 6 shows that the mining methods in this district are different from those in the Gray slope district. In the first place, the bottom bed was much thicker, and removal of so much coal to make room for packing rock from the divider was not necessary. Hence, a pillar of fairly good size was left in the bottom bed. During the first mining of the top bed, however, the coal was removed to the same distance as it was in the bottom bed; consequently the rock which was left on the rock pack beneath commenced to sag, and with no support of coal in the roof it broke, as the illustration shows, weakening the pillar and tending to bring on a squeeze. When new territory was opened up, this method was abandoned and the first system described was adopted. This system has the following advantages:

A minimum amount of the dividing rock has to be brought to the surface, and continuous support is given the upper bed as mining proceeds. The system becomes a sort of longwall system, advancing and retreating, and the greatest possible recovery results. It has been estimated that in the areas where this system has been used exclusively the recovery has been 95 per cent or more.

The next method described is second mining of an underlying firstmined bed.

EFFECT ON A I0WER BED OF SECOND MINING AN TJPPEB, BED

The anthracite region of northeastern Pennsylvania contains a number of coal beds separated by strata that range from a few feet to many hundreds in thickness. Where the parting is thick the first or second mining of a coal bed of medium thickness has a comparatively slight effect on either the underlying or overlying beds; but where the parting between the beds is anywhere from 6 to 20 feet serious complications can arise when an overlying or an underlying bed is first or second mined.

The selection of a method for obtaining the coal is very important where the parting approaches the lower limits mentioned above. Even first mining demands thought, particularly where first mining