THE NEWCASTLE COALFIELD. 243 



to the following matters in connection with the safe limit of 

 working coal under the ocean : 



(1) Character of the overlying strata, with special reference 

 to loose deposits of alluvium or beds of clay between the bed 

 of the ocean and the coal seam. 



(2) Presence of faults and dykes in the strata. 



(3) Dimensions of pillars to be left and the width of open- 

 ings to be made, or, in other words, the percentages of coal to 

 be left and worked respectively. 



(4) The utility of leaving coal next to the roof in some 

 cases. 



If there is a considerable bed of clay betw r een the body of 

 water overhead and the solid rock, this will tend to fill up any 

 joints and cracks, thus stopping infiltration. Should there 

 be, however, no clay, but only sand or shingle, then such a 

 stratum of loose material would be as bad as if the water 

 itself were immediately above the solid rock. 



After proving the presence of faults and dykes, since the 

 strata in their vicinity are disturbed, any passage approach- 

 ing them should be reduced in size as much as possible, and a 

 pillar of coal left next to the fault or dyke, the size of the 

 pillar depending on local circumstances. The main point in 

 working coal under large bodies of water is to lay bare as 

 little as possible any fault or dyke met with. When working 

 under tidal waters or under the sea, the minimum width of 

 pillars allowed is 8 yards, and the maximum width of bords 

 or other excavation is 6 yards. The 8 yard pillars must re- 

 main unwrought. The coal workings must be carefully sur- 

 veyed every three months, and a record kept of all dykes and 

 fissures met with. In one road of every pair of winning-olf 

 or leading headings, a bore shall be kept going 10ft. in ad- 

 vance, for the purpose of foretelling the presence of any fis- 

 sure, washout, open joint, fault, dyke, or otherwise, and all 

 winning-oif headings shall be driven at least 50 yards in ad- 

 vance of the working bords, when under tidal waters; or 100 

 yards in advance in the case of working under the sea. Coal 

 under the ocean should not be attacked until after a large goaf 

 has, if possible, been made by extensive coal-workings under 

 the mainland. The amount of cover of solid rock between 

 the coal seam being w r orked, and the body of water overhead 

 must be at least 120ft. The most accurate and trustworthy 

 information should be obtained, not only of the depth and cha- 

 racter of the sea bottom, but also of the strata overlying th' - 

 coal seam, which strata must be bored through, and proved to 

 have a minimum thickness of 8()ft. at the face of the leading- 

 headings as soon as they have been driven 100 yards in advance 

 of the working bords. After the first borehole has been com- 

 pleted, other boreholes must be put up in advance of it at 



