the slow or sudden uplift of the floor of the seam, and such like happenings.

A frequent accompaniment of severe crumps is a violent blast of air generated by the instantaneous displacement of large masses of rock material from the face, roof or up-thrust floor. The effect of such an air-blast may cause serious injury to men working at the face or in roads. Not infrequently the earth tremor generated by the crump is felt at the surface as a small local earthquake which may be registered by a seismograph several miles away.

The conditions in different mines vary greatly, and therefore the liability, magnitude and frequency of underground disturbances also vary. The depth of the mine, nature and inclination of the strata and especially the nature of the roof and floor of the seam, presence of faults, method of working, are all important factors to be reckoned with. There is also the question of gas accumulation in the coals and associated strata.

Conditions at Pendleton Colliery.-The natural conditions and method of working the Rams Seam at Pendleton Colliery where crumps in recent years have been particularly frequent and severe, are as follows:

1. Depth.-The Rams Seam, 6 feet thick, lies in that part of the Colliery where serious crumps have been experienced at a depth of over 3,000 feet, and at a depth of 3,550 feet where the crump of November, 1925, occurred.

2. Nature and Inclination of Strata.-The strata immediately overlying the Rams Seam consist of tough shale 35 to 50 feet thick interstratified with bands of ironstone. The floor beneath the seam is a warrant or shale about three feet thick with streaks of coal near the base, which lies upon the Little Rams Seam, 2 feet 6 inches thick. Under the Little Rams Seam is a floor of very hard grey sandstone rock.

The strata dip at about 1 in 3 to the South-South-West. 3. Presence of Faults.-Faults of small throw and generally with a southerly downthrow are frequently met with in the workings. They run in roughly parallel belts nearly with the strike, and consequently roughly parallel to the Levels in the mine and at right angles to the Brows.

The Cleat or main cleavage or joint planes in the measures, runs approximately north-west and south-east and makes an angle of 29 degrees with the strike or level course; while a secondary cleavage runs at an angle of 61 degrees to the main cleat-see Plate 5.

Since this report was first drafted, a recent paper on crumps in RhenishWestphalian Collieries has been consulted, in which it is stated that the crumps there are undoubtedly due to rock pressure and compression.-"Gebirgsschlage in Rheinisch-Westfalischen Stein-kohlenbergbau." W. Lindemann, "Gluckauf," 62, No. 10, 6th March, 1926, pp. 293-301; No. 11, 13th March, 1926, pp. 331-336 and Figs. 1-15.

followed consists of the driving of "brows" to the dip off which levels are turned to the right and to the left-the coal thus opened out being worked to the rise.

Coal, however, has been worked with the faces at right angles to the strike, i.e., " on end," particularly below the No. 14 West and the No. 15 East Levels.

5. Occurrence of Gas.-As a result of our investigations and on the evidence available, we are satisfied that firedamp is rarely present in the workings and we may state at once that we consider the issue of firedamp into the workings on the occasion of a crump is a consequence and not a cause of such an occurrence. The big red flash thought to have been seen by the witness Seabridge (see p. 4) was not due to the ignition of gas.

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Keeping in mind the foregoing conditions and reference being made to Plate 1, it will be found that the several crumps of which we have been given particulars, as well as that of November, 1925, have certain features in common, namely:

(a) All occurred at depths more than 3,000 feet below the surface;

(b) All except that marked (1) on the plate occurred when the coal was being worked

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on end ;

(c) All except that marked (3) on the plate occurred when working near a fault or break.

Apart from the direct evidence of the General Manager of the Colliery as to the restriction of crumps to the deeper parts of the mine, it would follow from the general considerations previously stated that at this, one of the deepest mines in the country, the pressure difference is such that when the stress equilibrium is disturbed the effects of stress relief must be exceptionally violent.

The strength and rigidity of the roof and floor of the coal seam are also of prime importance in any attempt to determine the cause of these sudden disturbances. At Pendleton Colliery, the roof over the Rams Seam is admitted by all who have had experience of working this seam to be exceptionally strong and tough. The comparatively small falls of roof that do occur affect the immediate roof only, but the roof above that appears to be of great strength.

With respect to the floor of the seam, one would be justified perhaps in calling it strong. For at such a depth, a weak or soft floor would be unable to withstand the great pressure, and would normally squeeze up or fracture. It is only relatively weak, as when it occasionally squeezes up or violently bursts up during a crump.

Our investigations in the mine at the working face and in the levels where the crump of November, 1925, occurred, and especially the results obtained by excavating holes in the floor down to and through the Little Rams Seam, have shown that the very hard grey rock below this latter seam was not broken

and burst up during this crump, but remained practically intact (see Plate 4).

It is probable, therefore, that this very hard stratum some 5 feet below the Rams Seam is a "competent competent" bed during these crump disturbances, and definitely limits the depth to which the disturbance penetrates.

But it is far otherwise with the relatively weak warrant earth, or smutty shale, between the Rams Seam and the Little Rams Seam. Our observations, confirmed by the holes dug in the floor during our investigations, showed clearly that it was this warrant and the underlying Little Rams Seam which were thrust violently upwards and were mainly responsible for the damage and loss of life. The roof remained intact, except for some small fractures, from which a small amount of gas escaped.

Cause of Crumps at Pendleton Colliery.-The vital question to be answered, if possible, is-What causes the sudden uplift of the floor, which is the chief and most destructive effect of the crumps experienced in this Colliery? It is not entirely a matter of depth with its consequent great pressure difference. For if it were so, we might expect crumps to happen universally below a certain critical depth. In the Rams Seam at Pendleton, the immediate cause would seem to be the relative weakness of the floor. It further appears that it is the floor at and near the working face which is especially liable to burst, and this because of an increased weight brought to bear upon the coal at the working face. A gradually increasing weight upon the coal at the face must come about consequent upon the working out of the seam, relieved of course, and to that extent diminished, by the artificial support of the roof by timbering and packs in the goaf, and relieved also by breaks in the roof near to the face, which have the effect of throwing more roof weight upon the packs in the goaf.

In any consideration of the great stresses which affect the strata in mines, such for example as the weight of the roof concentrated by cantilever action upon the coal at the working face, it is important to take into account the great effect produced by a sudden blow, as when some powerful compressive or bending stress is suddenly released. The "Bumps "which sometimes occur in working the Thick Coal of South Staffordshire are brought about by the sudden collapse of large masses of rock in the higher roof, where a space has been left between. the sagging immediate and softer roof and the stronger roof above. It is quite possible that a similar thing happens at Pendleton Colliery where portions of the more massive roof may suddenly fall upon the softer metal roof, which has already sagged in the goaf. Such violent impact would be sufficient to produce a great and momentary increase of weight on the coal face, and this sudden extra weight might be sufficient to overcome the resistance of the floor, which in consequence would burst up and produce a crump.

In this connection it may be noted that the shaley character of the floor between the Rams Seam and the Little Rams Seam would lend itself to a lateral shearing movement away from the coal-face towards the packs when the increased downward pressure on the coal-face is felt. The appearance of the chocks and props after the accident of November, 1925, was consistent with such a lateral thrust of the floor away from the coal-face.

There is another way in which a sudden and violent shock may be imparted to the coal-face, and that is by the sudden release of stresses induced by the bending or arching of the roof, the release being due to fracture some distance from the face. By this means the coal-face or pillar would suffer a powerful and instantaneous downward blow, which might produce a serious crump.

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Minor crumps which involve the sudden spalling or rating of large and small lumps and masses of coal from the coal-face are of frequent occurrence at Pendleton Colliery, and occasionally cause serious injury to the miner. Such occurrences, as well as the crushing and splintering of the body of the seam for some distance into the face, are part of the same general phenomena as the more severe crumps which are accompanied by a sudden uplift of the floor, and all are the result of the same group of causes.

It has been said already that the ground where the Rams Seam is being worked in the deepest part of the Pendleton Colliery, and where severe crumps have occurred, is traversed by a number of nearly east and west faults. It is necessary to inquire whether these faults are in any way a pre-disposing cause of crumps.

On general grounds it may be argued that the presence of such faults must lead to greater instability than would obtain in their absence. It is well known to geologists and seismologists that sudden slippages along old faults frequently take place, and that many earthquakes, small and large, in this country and abroad, arise from such slippages.

A sudden movement along the plane of fracture of one of the faults traversing the Pendleton Mine, involving it may be only a slight further dislocation, might be sufficient to disturb momentarily the stress equilibrium in the rocks, and so help to produce a crump.

Another way in which faults, it is suggested, may be a contributory cause was referred to in evidence by the General Manager of the Colliery. A section of unworked coal may be isolated by faults on one or more sides, with the result that there comes a time when so much roof weight is thrown upon the unworked remnant of the section, which is of the nature of an unworked pillar, that the critical point of floor resistance is reached, and a crump ensues. Whether this is the real explanation or not, it is significant that serious crumps have occurred in heavily faulted ground at Pendleton. In other coalfields abroad,

as for example in Nova Scotia, faults are believed to be a contributory cause of such sudden disturbances.*

To decide whether or not this is the real explanation of the effect produced by one or more faults bordering an unworked section or pillar of coal, it would be necessary to know the nature of the faulted ground, and whether the strata adjacent to the fault were in a state of lateral tension or compression, and further, whether the fault-plane is in the nature of a fracture similar to fractures which appear in the roof of a mine. But the vital question would be, as indicated above, whether the fault is still alive," and small slippages still taking place or liable to happen along the fault-plane.

Factors contributing to increased pressure at Pendleton Colliery. If we are right in concluding that it is the increased, and in some cases suddenly increased, roof pressure on the coalface which is the chief cause of crumps at Pendleton, at all events those severe crumps which involve injury and loss of life, we have to inquire as to all known factors which may contribute to such an increased pressure.

Apart from the baring of the roof and consequent removal of support in extracting the coal, how far is the weight on the coalface increased, directly or indirectly, by the inclination of the strata, by faults, presence or absence of cleat, and method of working?

With regard to the inclination of strata, which at Pendleton is about 1 in 3, it has been found that the liability to crumps in the Rhenish-Westphalian coal minest is greater when the measures are horizontal or nearly so than is the case when the measures dip at a considerable angle, although instances are cited where violent bumps accompanied by heavy falls occurred in inclined beds.

The facility with which the roof breaks behind the coal face is an important matter, and related to the method adopted in working the seam. It will be evident that where the solid roof can fracture easily and regularly as the coal-face advances, the extra weight thrown upon the coal-face by opening out will be continuously limited by such breaks. Therefore any practice which tends to produce such regular fracture and steady fall of the roof in the goaf near to the coal-face will conduce to safe working and tend to avoid crumps.

The experience at Pendleton Colliery accords with what we should expect, namely, that in working the coal to the rise, the roof tends to break more freely and regularly than when it

"Occurrence of Bumps in the Springhill No. 2 Mine of the Dominion Coal Company, Ltd." Report by G. S. Rice, Chief Mining Engineer, U.S. Bureau of Mines. + See note on p. 7.