the bottom of the working places dropping out. Accidents of this kind have occurred but fortunately with little or no loss of life. Of course, it is necessary to columnize the pillars rather carefully in work of this character, but when second mining is undertaken the real difficulty arises, especially where the underlying bed had been mined many years before the mining of the overlying bed began. First, in the older mine the pillars left were not large enough to sustain the increased weight put upon them by mining of the overlying bed, which is separated by only a small parting of rock. The pillars may have been strong enough to support the roof of the lower bed, but first mining of the overlying bed increases the height of a column without proportionately increasing its size to sustain the same weight. Therefore, if the column was designed to be just strong enough to sustain this weight, the added height weakens it, and the column is apt to fail.

MINING METHODS AT AECHBALD COLLIERY, GLEN ALDEN COAL CO.

Beds mined.—At the Archbald colliery of the Glen Alden Coal Co. near Scranton, Pa., the Lower Diamond and the Rock beds are separated by a 6 to 12 foot parting of the nature of "soapstone"; it is very weak and falls easily.

The Lower Diamond bed has the following sections:

Table 16.—Section of Lower Diamond bed in Archbald colliery

The Eock bed is 7 to 9 feet thick with no refuse, except about 2 inches of bony coal on the top. Samples of coal from these beds have the following analyses:

Table 17.—Analyses of samples from Rock and Lower Diamond bels

The Rock bed lies about 170 feet below the surface, and first mining was done about 25 years ago. The Lower Diamond bed was not considered of value at that time, so no thought was given to its mining in the first mining of the Rock bed. The pillars left were strong enough to sustain the roof of the Rock bed but were not intended to support it when their height would be increased by the mining of the Lower Diamond bed. The rock parting between these beds, being of the nature of "soapstone," would not give any lateral support, consequently when the Lower Diamond bed was mined the pillars had to be considered as being the full height from the bottom of the Rock to the top of the Lower Diamond bed, or between 18 and 25 feet high, their height depending largely on the thickness of the parting between the beds.

First mining methods.—Ordinary methods of mining and laying out the workings were used when first mining of the Rock bed was in progress, and when opening up of the Lower Diamond bed was desired great care was taken to see that the pillars were columnized with those in the underlying Rock bed. No matter how carefully the work is done, it is almost impossible to columnize pillars exactly. The pillars in the upper bed may be directly above those in the lower, or they may overhang a little on one side or the other so that their total strength is decreased by the amount of the hangover.

The accompanying map (fig. 9), which shows the Lower Diamond bed superimposed on the Rock bed, indicates that, generally speaking, the columnizing of pillars is exceptionally good, especially where modern methods of mining the underlying Rock bed were used. In these places pillar is over pillar, but where the older and more haphazard methods were used the columnizing is poorer.

Careful study of Figure 9 will show the hangover of the pillars mentioned in a previous paragraph; this hangover tends to weaken the pillars, as the load is not carried properly by the pillars below.

Second mining methods.—In the first mining of the Lower Diamond bed the greatest trouble was experienced when the soapstone floor broke loose and fell into the Rock bed below. Although this fall was not serious at the time, when second mining was decided upon the difficulties multiplied. Various methods of mining the coal from the bottom of these beds have been tried, but so far no satisfactory procedure has been devised. In 1917 a prominent consulting engineer was asked for advice. He suggested the following procedure: (1) Go to the beginning of the barrier pillar and take a 10-foot cut out of the pillars in the Rock bed, shoot down the parting rock between the two beds, timber the parting rock at the point where the break is desired, then shoot down the Upper Diamond bed and get through the parting rock to the Upper Diamond bed, take a 10-foot slice in the Upper Diamond bed, remove 10 feet from the Kock bed, break down the parting rock, retreating in this manner until the mine roof breaks; and (3) begin then again in the manner already described. The idea was to work on a number of pillars at the same time and to keep an even retreating face, so that when the mine roof broke the break would be along a definite line.

The theory was excellent, but unfortunately the first mining was unsatisfactory. The pillars were so weak that when the mine roof broke the break carried over and crushed the pillars ahead for 400 feet. In order to recover the coal, forepoling was necessary to reach the pillars in the Kock bed, which are of greater value than those in the Lower Diamond bed. When this system proved impracticable, another was attempted—that of putting up rock holes to the Lower Diamond bed, drawing back the pillars in the Lower Diamond bed, and then taking back the pillars in the Rock bed. Again the pillars proved too weak, and when weight came upon them through the removal of the pillars in the Upper Diamond bed a squeeze crushed the pillars in the Rock bed, carried for some distance beyond, and necessitated forepoling to permit recovery of the valuable pillars in the Rock bed.

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

In the last 20 years the art of mining has advanced by leaps and bounds. Mining methods have had to be worked out to meet conditions that were undreamed of in the past. Beds of coal that now are considered valuable were then deemed worthless, and mining was conducted without any thought of getting the coal from them, either then or later. This lack of foresight has made the recovery of coal much more difficult and expensive than it would have been if the beds could have been worked by modern methods from the beginning.

Beds mined.—At the No. 5 colliery of the Susquehanna Collieries Co., Nanticoke, Pa., the Twin beds in the No. 22 slope section lie with a parting of only 7 feet between them. The Top Twin was considered a better grade of coal, and first mining had been completed. Later, mining of the Bottom Twin was considered economically possible, and the bed was opened. Here, again, came the difficulty of opening an underlying bed that was separated from an overlying first-mined bed by a comparatively thin parting. At present, with the mining methods now in use, it is quite possible that both of these beds, if intact, would be mined simultaneously; but, as mining in the upper has been completed, only the lower bed

through the parting rock to the Upper Diamond bed, take a 10-foot slice in the Upper Diamond bed, remove 10 feet from the Rock bed, break down the parting rock, retreating in this manner until the mine roof breaks; and (3) begin then again in the manner already described. The idea was to work on a number of pillars at the same time and to keep an even retreating face, so that when the mine roof broke the break would be along a definite line.

The theory was excellent, but unfortunately the first mining was unsatisfactory. The pillars were so weak that when the mine roof broke the break carried over and crushed the pillars ahead for 400 feet. In order to recover the coal, forepoling was necessary to reach the pillars in the Rock bed, which are of greater value than those in the Lower Diamond bed. When this system proved impracticable, another was attempted—that of putting up rock holes to the Lower Diamond bed, drawing back the pillars in the Lower Diamond bed, and then taking back the pillars in the Rock bed. Again the pillars proved too weak, and when weight came upon them through the removal of the pillars in the Upper Diamond bed a squeeze crushed the pillars in the Rock bed, carried for some distance beyond, and necessitated forepoling to permit recovery of the valuable pillars in the Rock bed.

MINING METHODS AT NO. 0 COLLIERY, SUSQUEHANNA COLLIERIES CO.

In the last 20 years the art of mining has advanced by leaps and bounds. Mining methods have had to be worked out to meet conditions that were undreamed of in the past. Beds of coal that now are considered valuable were then deemed worthless, and mining was conducted without any thought of getting the coal from them, either then or later. This lack of foresight has made the recovery of coal much more difficult and expensive than it would have been if the beds could have been worked by modern methods from the beginning.

Beds mined.—At the No. 5 colliery of the Susquehanna Collieries Co., Nanticoke, Pa., the Twin beds in the No. 22 slope section lie with a parting of only 7 feet between them. The Top Twin was considered a better grade of coal, and first mining had been completed. Later, mining of the Bottom Twin was considered economically possible, and the bed was opened. Here, again, came the difficulty of opening an underlying bed that was separated from an overlying first-mined bed by a comparatively thin parting. At present, with the mining methods now in use, it is quite possible that both of these beds, if intact, would be mined simultaneously; but, as mining in the upper has been completed, only the lower bed