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exceeds two or three, and is, more often, much less. 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, bv 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 madre- 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