COAL-MEASURES. 



349 



Fig. 454.— Map of Anthracite Region of Pennsylvania (after Lesley). 



Faults. — It is plain, from what has been said above, that there is 

 an essential difference between a coal-seam and a metalliferous vein. 

 Coal-seams are conformable with the strata, and are therefore ivorked 

 wholly "between the strata. This would be a comparatively easy matter 

 if it were not for slips or faults which often occur, and sometimes make 

 the working unprofitable. In case of a fault, it is important to remem- 

 ber the rule already given on page 231, viz., that most commonly the 



Fig. 455. — Section across Yarrow Colliery, showing the Law of Faults (after De la Beche). 



strata on the foot-wall side of the fissure goes upward. In the following 

 section of Yarrow colliery it will be seen that all the slips follow this law. 



Thickness of Seams. — Coal-seams vary in thickness from a fraction 

 of an inch to forty or fifty feet. A workable seam must be at least 

 two feet thick. A pure, simple seam is seldom more than eight or 

 ten feet. Mammoth seams, such as occur in the anthracite region of 

 Pennsylvania, and in Southern France, are produced by the running 

 together of several seams by the thinning out of the interstratified shales 

 and sandstones. They are, therefore, almost always compound seams, 

 i. e., separated by thin partings of clay — too thin to form a good roof 

 or floor, and therefore all worked together. 



Number and Aggregate Thickness. — In a single coal-field, we have 

 said, the strata, including the coal-seams, are repeated many times. In 

 the South Joggin's section, Nova Scotia, there are eighty-one coal- 

 seams, though most of these are not workable. In North England there 

 are twenty to thirty seams In South Wales there are more than 100 

 seams, seventy of which are worked. In South Lancashire there are 

 seventy-five seams over one foot thick ; in Belgium 100 seams, and in 



