This system was extremely good for first mining and fairly good for second mining where the immediate roof was hard and strong and would not fall; but where the roof was at all weak the wide rooms and wide crosscuts tended to weaken the rock just above the roof, which would fall from rib to rib, filling the room with rock. Consequently, when second mining was attempted these places had to be cleaned up to make the coal accessible. This cleaning up was costly and the expense might readily be high enough to absorb all profit that might otherwise be possible in second mining by this system. For this reason the block system has been virtually abandoned in the anthracite fields.

ATTEMPT TO REOPEN HILLMAN BED

Second mining of anthracite often has been made difficult by the long time that elapsed after first mining was completed. The circulation of air through old workings causes deterioration of the roof and overlying rock, and many falls result, which fill rooms and gangways with loose material that has to be cleaned up before mining can be resumed. Such conditions are common in areas where the pillars left were so small that skips or slices can not be taken on the advance and the remainder of the pillar pulled on the retreat. Where the roof is of fire clay the difficulties are extremely great, for if fire clay falls and is moistened its bulk increases; moreover, falls of fire clay tend to extend over the top of a pillar, thus weakening the support of the main roof.

To open up territory of this kind removal of fallen fire clay from to allow recession of the fire clay over the pillar weakens the pillar greatly, and although a pillar may have been strong enough to permit skipping under normal conditions, the removal of a slice has merely the effect of reducing width of the coal pillar to that of the fire clay remaining above it.

This condition exists at the No. 7 colliery of the Susquehanna Collieries Co. at Nanticoke, Pa. Unfortunately, when the writer visited this colliery the Hillman bed was inaccessible. The coal was 6 feet thick, and over it lies a bed of fire clay 4 to 6 feet thick. This clay falls, and as it swells fills the openings in .the bed until there is only 2 or 3 feet between the top of the fallen fire clay and the main roof.

When this territory was first mined, 4 to 6 inches of bony coal was left in place to hold the fire clay, and no timbering was used. As the roof took on weight this bony coal broke and the fire clay fell, almost completely filling the openings in the bed. The map indicates that more than 50 per cent of the coal was taken in first mining.

In 1920 the company decided to attempt to reopen this bed to recover the pillars. An article in Coal Age,4 "Reopening of a thick flat anthracite bed," describes the work. A homemade mechanical loader of the belt-conveyor type was used in cleaning up one of the gangways for about 1,100 feet. This machine gave excellent results, although the repair cost was high, because the machine was built of scrap material.

While the roadway was being cleaned it had to be propped carefully and lagged so that the fire clay would not slip into it continually. As the map (fig. 47) shows, the roadway was driven so wide as to weaken the support that the fire clay had given to the pillars. Weight was thus thrown on the pillars, causing them to crack and spall. During the five-week suspension of mining in the latter part of 1920 this roadway crushed and no attempt has been made since to reopen it.

To skip the pillars on the advance and pull the remainder on the retreat was deemed impractical, as skipping the already small and weakened pillars would weaken them so that they would crush and be lost. The only possible method of reopening the bed successfully seems to be to make roadways up alternate rooms to their upper end and then pull them retreating as originally planned. The engineers of the company feel that the method used will be successful if the roadways are made narrower and only a small territory is opened at a time, as retaining the fallen fire clay in place is difficult and careful lagging is required.

This system was extremely good for first mining and fairly good for second mining where the immediate roof was hard and strong and would not fall; but where the roof was at all weak the wide rooms and wide crosscuts tended to weaken the rock just above the roof, which would fall from rib to rib, filling the room with rock. Consequently, when second mining was attempted these places had to be cleaned up to make the coal accessible. This cleaning up was costly and the expense might readily be high enough to absorb all profit that might otherwise be possible in second mining by this system. For this reason the block system has been virtually abandoned in the anthracite fields.

ATTEMPT TO REOPEN HILLMAN BED

Second mining of anthracite often has been made difficult by the long time that elapsed after first mining was completed. The circulation of air through old workings causes deterioration of the roof and overlying rock, and many falls result, which fill rooms and gangways with loose material that has to be cleaned up before mining can be resumed. Such conditions are common in areas where the pillars left were so small that skips or slices can not be taken on the advance and the remainder of the pillar pulled on the retreat. Where the roof is of fire clay the difficulties are extremely great, for if fire clay falls and is moistened its bulk increases; moreover, falls of fire clay tend to extend over the top of a pillar, thus weakening the support of the main roof.

To open up territory of this kind removal of fallen fire clay from to allow recession of the fire clay over the pillar weakens the pillar greatly, and although a pillar may have been strong enough to permit skipping under normal conditions, the removal of a slice has merely the effect of reducing width of the coal pillar to that of the fire clay remaining above it.

This condition exists at the No. 7 colliery of the Susquehanna Collieries Co. at Nanticoke, Pa. Unfortunately, when the writer visited this colliery the Hillman bed was inaccessible. The coal was 6 feet thick, and over it lies a bed of fire clay 4 to 6 feet thick. This clay falls, and as it swells fills the openings in .the bed until there is only 2 or 3 feet between the top of the fallen fire clay and the main roof.

When this territory was first mined, 4 to 6 inches of bony coal was left in place to hold the fire clay, and no timbering was used. As the roof took on weight this bony coal broke and the fire clay fell, almost completely filling the openings in the bed. The map indicates that more than 50 per cent of the coal was taken in first mining.

In 1920 the company decided to attempt to reopen this bed to recover the pillars. An article in Coal Age,4 "Reopening of a thick flat anthracite bed," describes the work. A homemade mechanical loader of the belt-conveyor type was used in cleaning up one of the gangways for about 1,100 feet. This machine gave excellent results, although the repair cost was high, because the machine was built of scrap material.

While the roadway was being cleaned it had to be propped carefully and lagged so that the fire clay would not slip into it continually. As the map (fig. 47) shows, the roadway was driven so wide as to weaken the support that the fire clay had given to the pillars. Weight was thus thrown on the pillars, causing them to crack and spall. During the five-week suspension of mining in the latter part of 1920 this roadway crushed and no attempt has been made since to reopen it.

To skip the pillars on the advance and pull the remainder on the retreat was deemed impractical, as skipping the already small and weakened pillars would weaken them so that they would crush and be lost. The only possible method of reopening the bed successfully seems to be to make roadways up alternate rooms to their upper end and then pull them retreating as originally planned. The engineers of the company feel that the method used will be successful if the roadways are made narrower and only a small territory is opened at a time, as retaining the fallen fire clay in place is difficult and careful lagging is required.

BTJGGY-ROAD MINING

Before drag scrapers, shaking chutes, and longwall conveyors were introduced, it was customary, where the coal was low and the bed pitched too steeply to permit mine cars to go to the working face and not steeply enough for the men to push the coal on sheet iron, to use buggy roads to some extent. Even at pi'esent a very large num

ber of buggy roads are in use, and new ones are being opened. Buggies are small, light, low mine cars, of various sizes, that will haul 500 to 1,000 pounds of coal. These cars are loaded with coal at the face, pushed by hand to the main gangway, and preferably dumped directly into the mine cars, or if not, onto a loading platform from which the coal is shoveled into the cars. Small room hoists are some