M I N E. 



323 



Hoof, of 

 cut. 



Mini of larly when of a large size. The peculiar singularity 

 Co*l. attending this trouble, is the uniform twisted texture 

 ^'V 1 "' ' and alteration which are found in the coal immediately 

 under it, without any mixture of the stone which com- 

 poses the pot-bottom. 



The Roofs of coal are formed of all varieties of the 

 coal strata, as may be seen from the examples of strati- 

 fication in the preceding part of this treatise. As roofs 

 they are of various quality, from good to very bad ; a 

 roof is reckoned good which keeps entire, not only 

 while the coal is working, but for a considerable time 

 after. Sometimes a stratum of roof-stone next the coal 

 comes down along with the coal while working, named 

 a following, leaving a firm roof-stone above; at other 

 times a stratum or two of the roof-stone have to be 

 taken down shortly after the coal is wrought : this is 

 also termed a following; and while the miner works 

 under it, he supports" it by prop-wood for his own 

 safety ; but these are not reckoned tJanfrerous roofs. 



A bad roof is that, which, having little tenacity in 

 itself, cannot be kept up either in boards, rooms, or in 

 narrow mines, even by the aid of prop-wood ; it has 

 therefore to fall in the waste, care being taken to keep 

 it up along the wall-faces ; and the iron rail-roads are 

 in some bad roofs laid upon the top of the fallen strata. 

 The roofs which are most dangerous to the workman 

 are those where the roof-stratum is thick, with rmny 

 open cutters in it, or where there arc irregular beds, 

 thick in the middle and wedge-shaped towards the 

 edges. These roofs give no warning before they fall, 

 by which the danger is much increased. The oramary 

 way in which a miner tries the roof to be safe or nut, 

 is to strike it gently with the side of his pick ; if the 

 ound produced is sharp and clear, he concludes that 

 the roof is good, but if the sound is obtuse and hollow, 

 he is certain that the roof U bad and dangerous. ! le 

 may, however, be deceived in trying a roof composed 

 of a thick stratum, as it will produce a clear sound 

 though ready to fall, owing to the cutters with which 

 it is intersected. 



Having stated the effects of dikes, slips, and hitches, 

 as to the dislocation of the strata, the i 

 ed by them upon the coals and strata adjoining have 

 now to be noticed. 



'.Vben coals approach dikes composed of sand-stone, 

 heterogeneous mixture of rtones, water, gravel, quick- 

 sand, or clay, we seldom find any great change in the 

 composition of the coal, exce, 



dcred a little softer or harder, with t'.c natural fissures 

 occasionally very open, and filled with sulphate of 

 lime and pyrites, or with jan.-l or clay when close to 

 dikes of this composition. The strata adjoining are in 

 some cases rendered flatter than in the general coal 

 field ; in other cases the dip and rise are greatly in- 

 creased, and the line of level frequently altered. U'e 

 have, however, to notice a very remarkable and decid- 

 ed change in the stratum or bed of coal, as it appr 

 e dikes composed of greenstone, b.isalt, or porphyritic 

 clay, dikes of greenstone or of basaltic rock, (the com- 

 mon blue whiintoiie of Scotland,) are much more com- 

 mon in the coal-fielil of Scotland than the porphyry 

 dikes ; and their i fleets are well known, as they arc fre- 

 ;tly met with in several of the mining districts. 

 The first indication perceived in approaching one of 

 Ibese dikes, is, that the coal begins to lose its common 

 brightness in the fracture, and assumes a dull black ap- 

 mce, but still retains iu common qualities of good 

 burning coal, with very little difference; but every foot 

 of advance towards the dike, the coal becomes worse. 



Coali ad- 

 joining 



mm, 



loses every appearance of common coal and all its qua- Mines of 

 lilies ; the texture of the coal is lost, and the whole , *l oa1 ' 

 stratum of it becomes a mass of black stoney matter, as ^"""^ """' 

 before described, when treating of troubles appertain- 

 ing the bed of coal, under head No. 10. Dike Coal. Ad- 

 joining to the dike, the cavities of this coal are sometimes 

 filled with a soft yellowish cliy, having an unctuous feel; 

 this clay is also sometimes found in the fi c sure betwixt 

 the coal and the face of the dike, when the dike is cut 

 through. The coal on the other side is found exactly 

 changed as in the approach to the dike before mention- 

 ed, viz. hard stoney, and cellular next the dike, and 

 these qualities gradually disappearing till the coil as- 

 sumes its common lustre, with all the qualities of good 

 coal, as found before the deteriorating symptoms ap- 

 peared on approaching the dike. 



When coals appror.ch slips, a variety of changes sre r oaUad. 

 found, and they are very seldom met without syniptomsJ'"' D 8 

 appearing of their approach, particularly if they are of*" 1 * 

 considerable magnitude. The most common symptoms 

 of their approach are, numerous hitches, fwcllin. 

 the roof, sudden risings or wavings of the pavement, 

 the coal much stronger in its texture than common, 

 and more difficult t > work ; at other times much softer 

 than usual, and this r oftness at times increases to such a 

 degree as to loer.il firmness of ttxti: iv. In this state it 

 if soni' c co.-il dust mixed with small cubical 



pieces of pood coal ; at other times it is found uncom- 

 monly friable, somewhat like soot The occurrence of 

 iron pyrites is very common, in some instances filling 

 fissures and cavities, having a beautiful 

 and crystallized, some of the cry-tals forming cubes 

 re than half an inch in site, and finely laminated ; 

 tances, the pyrites are so run into and en- 

 twisted with the coal as to form a very heavy mass of 

 singular texture. \Vc also find the sulphate of lime very 

 abundant near to slips, filling up the numerous fissures 

 of the coal, and entwisted with it coals of the soft 

 and of the soot jr quality above mentioned; where the 

 pyrites and sulphate of lime abound, the coals are of no 

 use as fuel. An increase of water from the fissures, as 

 a dike or slip is approached, is also common ; and when 

 the slip is found, very great feeders of water sometimes 

 occur. If the coal i< dry, inflammable air (if found ge- 

 nerally in the coil-field) comes off in great quantity; 

 and there are instances of inflamm..' , found 



at dike or slip though never seen before in the dis- 

 trict. 



Sand-dike*, or gashes containing quick .sand, are Sand dikes, 

 dangerous from the quantity of water they contain, 

 which is found in some instances so great as to drown 

 and destroy the colliery, if not prevented by powerful 

 means. 



We have now to describe the general effect produc- 

 ed upon the coals and accompanying strata by dikes 

 and slips, in order to shew how coals wave, or are 

 spread over a great district of country ; and thii ia es- 

 sentially necessary before attempting tu describe the 

 modes which have been adopted in searching and bor- 

 ing for coal. 



Although, as before mentioned, there are instances Effects of 

 of coal-fields of a circular bason form, and more com- dikes ind 

 monly of a long elliptical shape, yet tli r- .ire few in- slip* on 

 stances, compa; f this entire form, either in the eol-fleld. 



ri^'ht or inverted position ; yet we (eckon that the rn- 

 tirc bason form is the true and complete form of a coal- 

 field, and that the general form of coal-fields are nor. 

 tions of this bason shape, produced by the naiur.il ef- 

 fects of dikes and slips, of which the following detail, 



