FAULTS AND DYKES. 



105 



per square mile. The thickest bed of coal is nine feet ; in some 

 parts there are sixteen seams of ironstone. The strata of this vast 

 coal-field are deeply cut through by valleys, and are much broken 

 by faults, and the quality of the coal varies greatly in different parts 

 of the field. 



At the Clee Hills in Shropshire, the breadth of some of the coal- 

 fields is not a mile. At Ash by Wolds in Leicestershire, in the central 

 part of the field at e, Plate IV. fig. 2., the main bed of coal is work- 

 ed at the depth of two hundred and forty yards ; but by the bend- 

 ing and rise of the strata, the same bed comes to the surface at 6, 

 about three miles distant. The depth of coal strata, from the incli- 

 nation or bending of the strata, differs much in the same district, as 

 will be evident from what has been stated, and from an inspection of 

 the last-mentioned figure. Some coal-fields extend in a waving 

 form over a district. 



On the eastern side of England, the strata generally decline, or, 

 in the miner's language, dip, to the south-east point : on the western 

 side of the strata they are more frequently thrown into different and 

 opposite directions, by what are called faults and dykes. 



A fault is a break or intersection of strata, by which they are com- 

 monly raised or thrown down ; so that, in working a bed of coal, the 

 men come suddenly to its apparent termination. A dyke is a wall 

 of mineral matter, cutting through the strata in a position nearly ver- 

 tical. (See Plate IV. fig. 2. and 3.) The name dyke is originally 

 derived from our Northern neighbors : it signifies a wall. The 

 thickness of dykes varies from a few inches to twenty or thirty feet, 

 and even yards. The dykes which intersect coal strata are compo- 

 sed of indurated clay, or more frequently of basalt, and will be par- 

 ticularly described in the following chapter, in some coal-fields the 

 strata are raised or thrown down on one side of a dyke one hundred 

 and fifty yards or more ; and the miner, after penetrating through it 

 (see Plate IV. fig. 3.), instead of finding the same coal again, meets 

 with beds of stone or clay on the other side at e : hence he is fre- 

 quently at a loss how to proceed in searching for the coal which is 

 thus cut off. If the stratum of stone e be the same as any of the 

 strata which were sunk through in making the pit or shaft g g, it 

 proves that the bed of coal on the other side of the fault is thrown 

 down, and he can determine the exact distance between that stratum, 

 and the coal he is in search of. But if the stone is of a different 

 kind to any which was above the coal he is working, he may be cer- 

 tain that the strata on the other side of the fault are thrown up, but 

 to what distance, if the under strata have not been previously per- 

 forated, can be ascertained only by trial. It frequently happens, 

 however, that two or more strata of stone or shale, at different depths, 

 . are so similar in their quality and appearance, that it is impossible to 

 distinguish them : in such cases, it is necessary to perforate the stra- 

 tum, to ascertain its thickness, and examine the quality of the strata 



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