320 The Supply and Waste of Goal. 



most districts, however, a two-foot seam, if of fair quality, and 

 not accompanied by very indifferent roof and floor, can be 

 worked, and thus all seams of two feet and upwards are gene- 

 rally calculated among the available coals of a district. 



On the other hand, very thick seams involve waste, especi- 

 ally when the roof above the coal is weak, for then coal, often a 

 foot thick, is left, and ultimately lost. In very thick seams, 

 where the coal is of good quality, the works are often conducted 

 in a very wasteful manner, and thus seams from three to six or 

 eight feet thick are perhaps the most profitable, and those least 

 wastefully worked. The number of workable seams in a coal- 

 field varies exceedingly : in the Newcastle coal-fields it is six- 

 teen, in the Lancashire seventy-five. 



3. Depth of workings. In most coal districts in England 

 the coal seams, although they may once have approached the 

 surface, have long since been worked out to all small depths, 

 and everything that can be got by horizontal tunnels or drifts 

 into a hill-side has been already removed. Pits or shafts have 

 then to be sunk, at a cost and risk varying according to cir- 

 cumstances, and varying so greatly as to give a character of 

 speculation to coal miuing that would not otherwise belong to 

 it. When the coal measures (the rocks known to belong to the 

 coal-bearing series) are at the surface, or only covered by gravel 

 and soil, the depth of any particular seam can be calculated 

 pretty closely, especially in a district where there are many 

 pits already sunk. Where this is not the case, and rocks of 

 newer geological date have to be pierced, there is, however, 

 great doubt. This arises from the fact that the exposed surface 

 of the coal series, as of other rocks, has been worn down by 

 air and water, and pared off to a great but unknown 

 extent, during the lapse of the ages that have passed away 

 since they were formed; whereas in those cases in which the 

 measures are protected by a newer rock, they have escaped 

 this, and are probably much thicker. There is also in such 

 cases a possibility of disturbances having altered the dip or 

 tilt of the beds, though generally the deeply-covered beds are 

 more likely to be horizontal. 



The present limit of workings in coal mines is not much 

 more than 2000 feet. At that depth in England the tempera- 

 ture is already so high as to be inconvenient, and it is at least 

 probable that, at greater depths, it would be still hotter. It has- 

 been found possible to work in mines where the temperature is at 

 least fifteen degrees of Fahrenheit higheV than in the celebrated 

 Monkwearmouth pit; but this is by slave labour, and might be 

 impracticable in England. At any rate, it is not likely that, 

 even with improved contrivances for lighting and ventilation, a 

 greater depth than from 3000 to 4-000 feet will bo attained in 

 coal mines. 



