492 Mr. Logan on the Underclay 



texture, yielding in most cases a very good fire-clay*. In many parts of the 

 coal-deposit of South Wales, it is tough, though not very hard, when fresh cut, 

 but on exposure to the weather it slacks, splits, and crumbles into a greyish mass. 

 It is occasionally quite black, from an admixture of carbonaceous matter, and is 

 then sometimes sufficiently consistent to resist the effects of weather ; and under 

 some of the lowest seams of coal between Swansea and the Bury river, it yields a 

 hard, durable, fine-grained, siliceous stone, very much resembling quartz rockf. 



It is not, however, by the mineralogical composition that these beds are so 

 thoroughly marked, for they not only vary considerably, but many strata oc- 

 curring in other parts of the coal-measures, are precisely identical in mineral con- 

 tents. The grand distinguishing feature of the underclays is the peculiar character 

 of the vegetable organic remains : these are always of one kind {Stigmaria Ficoides), 

 and are so diffused throughout every part of the bed, that by their uniform effiect 

 alone the clay is readily recognized by the eye of the miner. 



The beds, varying in thickness from six inches to more than ten feet, and 

 averaging about three, are crossed and penetrated vertically, horizontally, and ob- 

 liquely, by a confused and tangled collection of long, slender, fibrous casts, with a 

 thin coating of carbonaceous matter. These fibres are sometimes cylindrical, 

 though generally flat, and are usually about a quarter of an inch in breadth ; but 

 they are very often traceable to a junction with a stem or branch, sometimes flat- 

 tened, sometimes not, and varying in diameter from about two inches to half a foot. 

 From this stem or branch, which is usually of considerable length, and always lies 

 in the plane of the bed, but rather nearer the top than the bottom, the fibres radiate 

 in all directions, and take such a tortuous irregular course that it is impossible to 

 follow any one of them to the natural termination, though it is easy to see that the 

 range is very considerable. This fossil, the Stigmaria Ficoides, taking for granted 

 that the slender fibrous impressions belong always to it, so completely fills every 

 bed of underclay, that it is not possible to cut out a cubic foot which does not con- 

 tain portions of the plant. It is also worthy of remark, that specimens with the 

 fibres emanating from the stem or branch are to be discovered only in the under- 

 clay. 



It is proper, however, to state, that such specimens, even if they did exist in other 



* It usually makes a good fire-clay when free from nodules of ironstone, which however are generally 

 disseminated in great abundance through it. 



f Not far from Penclawdd, on the Bury river, there is a seam of coal called the Farm Seam, the bed 

 beneath which is a grit or sandstone, and I should think sufficiently durable to be used for building pur- 

 poses. On first seeing it I was disposed to consider it an exception to the general rule of underlying beds, 

 but on breaking it across the plane of the bed I found it penetrated, like all the rest, by the long fibrous 

 processes of the Stigmaria, though not quite in the usual abundance. 



