COAL. 301 



exceeds two or three, and is, more often, much loss, 

 it is frequently also variable in the same bed; and 

 thus particular strata become extenuated till they dis- 

 appear. Sometimes these beds have a schistose struc- 

 ture; in other cases they are massive; and, in both, 

 they are often divided, like the argillaceous schists, by 

 joints more or less parallel, at angles to the planes of 

 their stratification. Where beds of coal have assumed a 

 more regular prismatic or columnar structure, this has 

 occurred only in the vicinity of trap, as I shall soon 

 notice. But in Glamorganshire, where this rock is 

 not found, specimens occur, of a singular concretion- 

 ary structure, presenting fibrous projecting ridges 

 disposed in a manner precisely resembling the inadre- 

 porite called brainstone. 



If single beds of coal sometimes occur in one place, 

 they are more frequently repeated in alternation with 

 the rocky strata by which they are enclosed. Such 

 repetitions have been known to amount to thirty and 

 upwards, as in Derbyshire and Northumberland; but 

 they are seldom so numerous. Alternations extending 

 from three or four to twelve, are more frequent; and, 

 in these collections of strata, the beds are, not only 

 unequal in thickness, but very different in quality; so 

 that, either from this or their insignificant quantity, 

 it rarely happens that more than two or three, even 

 in a considerable series, are worth working. 



It must have been already understood, from former 

 observations, that the coal series is not every where 

 found among the secondary strata, however steady its 

 place may be where it exists, but that it occurs in 

 distinct tracts often widely separated from each other. 

 These are known, technically, by the term coal fields, 

 and they vary in their characters in different places; 

 not only in their extent and in their depth, but in the 



