that to work the bed with a rock cover of probably 50 feet will prove possible, as has been found possible in other collieries.

Suggestions for second mining.—At present there is no intention of conducting second mining in this territory, for there is considerable danger that a roof break extending upward to the water-bearing sands of the buried valley might cause the flooding of the mines. The possibility of second mining this territory has been considered, however, and one method suggested by the company engineers is that rock holes be driven through the parting from the Bottom to the Top Twin bed.

As the parting spalls readily, these rock holes would have to be carefully timbered along the edges, possibly with cogs or rows of props; then, second mining would start from the upper end of the chamber in the Top Twin, and the Bottom Twin would be worked simultaneously. The coal from the Top Twin would be sent down through the borehole to the waiting mine car or sheet-iron chute underneath, and the coal from the Bottom Twin would be shoveled either into the mine car or to the chute and taken to the gangway.

The company engineers have also advanced the idea that it may be possible to shoot down and gob the parting rock between the two beds and take a slice from the pillar advancing, then to stow the parting or enough of it in this space so that a roadway can be cleaned up in the.room. The coal in the Top Twin would thus be made accessible and the recovery of coal from both pillars permitted. Recovery of the coal from the pillar in the Bottom Twin on the opposite side of the room would necessitate clearing up short roads across the room to each one of the pillars. These rock holes or roadways would need to be placed in alternate rooms only, as two pillars could be recovered for each roadway or set of rock holes.

Of course, in work like this great care must be taken in order that the utmost amount of coal may be recovered consistent with furnishing ample support to the roof, that no breaks endangering the colliery may occur, and that the place will be left in such a condition that more of the coal will be obtained by second mining if a method that will give enough final support to the roof is found.

ROCK FILLING TWO BEDS MINED SIMULTANEOUSLY

Beds mined.—In many parts of the anthracite region the parting between two beds becomes very thin, too thin to allow either of the beds to be mined separately; then both have to be taken at once. Such a condition exists in the No. 17 slope district, road 415 of the colliery, where the Top and Bottom Skidmore beds are separated gives a section of these two beds from the top downward. The roof, a hard sandstone, extends upward 160 feet to the overlying Bennett bed.

Table 20.—Section of Skidmore beds in No. 17 slope district

The total thickness of the two beds is about 10% feet or, if the dividing rock between the two beds is included, 12% feet. The coal is rather hard and pitches about 8°. Figure 16 gives a map of the section.

Mining methods.—In mining this coal the ordinary room-and-pillar method of the anthracite region is used. The rooms are driven on

Figure 16.—Map of Lower Skidmore bed in No. 17 slope section, showing barrier pillar and second mining (hatched areas) where Top and Bottom Skidmore beds are mined as one

60-foot centers in the usual manner, except that in driving the rooms up the pitch only the bottom bed and the parting rock are taken. The parting rock is gobbed to one side or the other, and when the gangway above is reached the top coal is shot down and is loaded into the mine cars. When a room has been completed, rock is dumped into it. If necessary, the rock is shifted by hand, so that it can be thoroughly packed to the roof to give the necessary support. (See fig. 17.)

When the rooms have been driven to the limit and the roof has been brought back as robbing begins, a pillar is started from the lower gangway through the center of the pillar until it reaches a point about 20 or 30 feet from the gangway; then a cut is made over to the nearest room on the right, and a skip (thin slice) of the bottom coal and the parting rock is taken from this point upward. Between the adjoining room on the right and the skip taken along the edge of the pillar a very thin pillar of coal is left to prevent the rock packed into the adjoining room from running into the place where the men are working. When the upper gangway is reached, the rest of the pillar is pulled back to the lower gangway, leaving another very thin

^Rock packing—^Pulling back'top

Figurb 17.—Details of method of mining and rock packing in the Top and Bottom

Skidmore beds

pillar between the room on the left and the place where the men are working. Thus, in this type of mining, virtually all of the coal is won except some top coal which may fall in the gob and the thin pillars that are left between the adjoining workings to keep the loose rock in place. These pillars are rarely more than 2 feet thick, in some places much less. Even if they average 2 feet, the amount of coal lost would be only about 4 per cent. Probably even less coal is lost in the gob. So far no pillar coal has been lost in robbing of this type where rock filling is used as roof support.

EFFECT ON AN UPPER BED OF MINING A I0WER BED

In the anthracite region the multiplicity of beds complicates coal mining somewhat, particularly when the parting between two beds is comparatively thin. Where the parting is thin, it is often possible to mine the two beds simultaneously; but as the thickness of the parting increases, the possibility of simultaneous mining decreases until at length the mining of both beds at the same time becomes economically impossible.

Just what this thickness may be depends entirely on the character of the parting, the thickness of the beds, and conditions at the colliery. Where the two beds are mined as one, the difficulty is lessened; but where they have to be mined as two separate beds the difficulty increases. However if both beds are virgin and are mined at approximately the same time the condition is handled much more easily than where one bed has been partly or completely mined and the other bed is mined much later.

This condition prevails to a large extent in the anthracite region, and the difficulties are much greater than they should be, because engineering in the earlier days of mining was poor. Much of the surveying done years ago was not as precise as that done to-day and consequently can not be entirely trusted; the general mine map may be reliable enough, and yet not accurate for individual sections of the bed. For example, the map may show a pillar of a certain size at a specified point. When the territory is reopened, the pillar may be there and may be of the size named, but it may be a few feet one way or the other from the point shown on the map.

Therefore, when it is desired to columnize the workings in two beds (one of which was worked years before) and the maps are inaccurate, considerable difficulty arises in getting gangway over gangway, room over room, and pillar over pillar. As a result, the room in one bed may overlap the pillar in the other bed, and so on.

MINING METHODS AT NO. 5 COLLIERY, SUSQUEHANNA COLLIERIES CO.

Beds mined.—At the No. 5 colliery of the Susquehanna Collieries Co., Nanticoke, Pa., the two George beds are separated by a 12-foot parting. The Bottom George bed is 4 feet thick and has a roof of bastard sandstone and fire clay, which forms the bottom of the Top George bed. The interval ranges from 12 to 20 feet. Table 21 is a section of the Top George bed.

Table 21.—Section of Top George bed in No. 5 colliery