300 COAL. 



bnted to the nature of their peculiar pursuit, and to 

 their limited geological knowledge, or their ignorance 

 of the disposition of the other stratified substances in 

 nature. The philosophical geologist views, in them, 

 examples only, of general laws which have influenced 

 the forms and dispositions of all the strata of the 

 earth. 



As the coal strata may be simply inclined, so they 

 are frequently curved : a common fact in all rocks, 

 but often of great value in a practical view, as restor- 

 ing to the surface those beds which, had they been 

 prolonged in the planes of their first inclination, would 

 have plunged beneath the reach of the miner's opera- 

 tions. Such curved deposits sometimes produce sin- 

 gle basins, under the same analogy ; the central parts 

 forming a general concavity, and the edges appearing 

 in various places around the circumference. 



Similarly, they present continuous undulations, 

 more or less marked, accompanying, as might be ex- 

 pected, the general undulations of the associated rocks. 

 From this cause, or from mere disturbance^ they are 

 sometimes found disposed in convergent or divergent 

 strata; offering circumstances of great practical mo- 

 ment wherever they occur. Of similar importance 

 are the ordinary fractures, to which the beds, in com- 

 mon with the rest of the strata, are liable. As is usual 

 among other strata, such fractures are attended by 

 shifts ; these being either simple, or accompanied by 

 veins of other materials. In practice, they produce 

 the sudden elevation or depression of the stratum in 

 which the work is carried on; the consequences, in 

 either case, being generally a source of much labour 

 and expense. 



The thickness of a coal stratum varies, even from 

 less than an inch, to ten or twelve feet; but it rarely 



