system of mining in which the distinguishing feature is the winning of 50 per cent or more of the coal in the first working. The coal is mined in rooms sepa rated by narrow ribs or pillars. The coal in the pillars is won by subsequent working which may be likened to top slicing in which the roof is caved in successive blocks. The first work ing in rooms is an advancing and the winning of the pillar coal a retreating method. (See Fig. 2). The rooms are driven parallel with one another, and the room faces may be ex tended parallel with, at right angles, or at an acute angle to the dip. When applied to metal mines, with a good cover, as much as 75 to 85 per cent of the ore is removed, leaving pillars 10 to 20 feet in diameter; wherever possible, pillars are left in the poorest ore, hence their position is more or less irregular as compared with the pillars left in coal mining. This method is applicable to flat deposits, such as coal, iron ore, lead and zinc, etc., which occur in bedded deposits. Modifications of this method are County of Durham system; Double-entry room and pillar mining; Double-room system; Double-stall working; Heading and stall; Pil lar and stall; Post and stall; Room and stoop; Single-entry room-and-pillar mining; Single stall working; Square work (South Stafford shire thick seam method); Stall and breasts; and Triple-entry room-and-pillar mining.
Longwall Method.—A system of working a seam of coal or ore in which the whole bed or seam is taken out and no pillars left, ex cepting the shaft pillars, and sometimes the main-road pillars. Longwall advancing con sists in mining the coal outward from the shaft pillar and maintaining roadways through the worked-out portion of the mine. (See Fig. 3, in which the arrows indicate the direction of advance and points to the working face). Longwell retreating, first driving haulage roads and airways to the boundary of a tract of coal and then mining it in a single face without pil lars toward the shaft. The longwall method is usually applied to coal mining and occasionally to flat ore deposits, as iron, etc. This system of mining is also known as Longwork, Shrop shire method, Combination longwall and Not tingham or Barry system.
Millin?.—A system of mining originating in the combination of open cut and underground mining, where the ore is mined in open cut (surface workings) and handled underground. It is underhand stoping applied to large de posits, wherein the ore is mined near the mouth of winzes or raises and dropped by gravity to working levels below for transportation to the surface. Sometimes called method. (See Fig. 4).
Caving System.—A method of mining in which the support of a great block of ore being removed, it is allowed to cave or fall, and in falling is broken sufficiently to be handled; the overlying strata subsides as the ore is withdrawn. Two important variations are Top slicing and cover caving; Top slicing combined with ore caving.
Top Slicing and Cover Caving.—The im portant feature in this method is the working of the ore body from the top down in successive horizontal slices that may follow one another sequentially or simultaneously.
The whole thickness of the slice is worked and the ore broken by over head or under hand stoping in each unit. It is a retreating method, as the overburden or cover is caved after mining a unit. The longwall, the pillar robbing in both room-and pillar and bord-and-pillar methods of mining coal are essentially the same in principle as top slicing. The principal difference is that a single slice only is worked in these methods when applied to coal mining. There are two modifications: top slicing by drifts and top slicing by rooms.
A timber mat is used as a cover in almost all cases. The method is ap plicable to thick or massive ore de posits. Other terms used for this system are Caving system ; Crosscut method (combined with removal of pillars) ; Horizontal slicing descend ing; Mining ore from top down; Panel slicing; Prop slicing; Remov ing pillars and allowing roof to cave; Slicing !under mats of timber in panels; Square-set slicing; Top slic ing and caving; Transverse slicing with caving.
Top Slicing Combined with Ore Caving.— In this method the ore body is worked from the top down in successive slices. Instead of taking the full height of the slice, only the lower part is taken and the upper part is caved. After removing this portion of the ore, the cover is caved. A timber mat is used in most cases to separate the broken cover. from the ore to give safety. Also known as Caving system, Sub-drifting and caving, Sub-slicing, Slicing under ore with back caving in rooms, Sub-level caving and Sub-level slicing.
Ezplosives.— In all methods of mining, ex plosives are essential. The maximum amount of explosives is used in stoping and room-and pillar methods, while a minimum amount is used in the caving system and its various modi fications, due to the fact that much of the ore is loosened by caving of the overburden. The importance of the mining industry is further shown by the fact that 75 per cent of all ex plosives used in the United States are used in the mining industry (not including quarries). The enormous production from the mines in the United States would not be possible without the use of large amounts of explosives. The total production of explosives in the United States in 1917 was as follows: Black powder, 277,118,525 pounds; dynamite and high explo sives other than permissibles, 262,316,080; per tnissibles, 43,040,722. Of the black powder, 85 per cent was used in coal mining; of high ex plosives, 10.8 per cent; and of permissibles, 76.5 per cent. Other mining consumed 3.4 per cent of the black powder, 50.4 per cent of high explosives and 14.2 per cent of permissibles. In the production of coal an explosive is re quired which will not shatter the coal and at the same time not produce too long a flame nor too much heat and thus ignite gas and dust with which it might come in contact. This led to the introduc tion of so-called safety or short flame explosives in the coal mines of Pennsylvania at Johnstown in 1902, and since that date their use has increased rapidly. The term «permissible explosives" was coined by Dr. J. A. Holmes in 1908, and in 1909 the Federal Bureau of Mines established a testing station at Pitts burgh to determine the permissibil ity of explosives that are to be used in coal mines. This work is still being carried on in connection with the investigation of mine explosions to determine the nature and char acter of an explosive that will meet the needs of the various branches of mining, and especially to increase safety in coal mines. Both the im proper use of explosives and the use of improper explosives have resulted in coal-mine disasters. The Fed eral Bureau of Mines has brought about what is little short of a revolution through the introduction of new types of low-tem perature, quick-flame powders, designated as «permissible explosives, ° for use in the more dangerous coal mines of the country. In the bituminous coal mines of the United States the fatality rate per 10,000 men employed has been reduced from 3.39 in 1903 to 0.80 in 1917, largely as a result of the use of permis sible explosives. In metal mining a different type of explosive is needed, namely, one which will shatter the hard mineral formation, and for this purpose dynamite of different grades is employed. Black powder, by reason of its slow heaving action, is used to a certain extent in the iron mining districts, where steam-shovel work is carried on, and where it is necessary to loosen large masses of soft or friable ore.