Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/34

12 a quarter of an inch thick, with an underclay. This ring of coaly matter thickens northward, so that at Hafod-y-bwch, it forms an impure coal five inches thick.

Above this group we reach a thin deposit of blue shale. This is succeeded by a fire-clay four feet thick; and this, in its turn, is surmounted by a coal-seam sixteen inches thick, of fair quality. This coal has a hard grey shale roof, which is a perfect storehouse of plant-remains: a brief attempt was recently made to work the coal; and in doing so a fine calamite stem, twenty feet long and from twelve to eighteen inches in diameter, was exposed in the roof shale. Pecopteris (Bucklandi?) abounds; so also do Stigmariæ and Lepidodendra of various species. There are also Asterophyllites, Neuropteris (cordata), Araucaria, Stembergia and other plant-remains. In driving a road through this shale some distance above the coal, there was found an erect trunk of Calamites or Calamodendron gigas, which measures eighteen inches across the base, and about six feet in length of which was excavated and is preserved. This specimen retains portions of the outer bark in a carbonized state. These fossiliferous shales are covered by a hard grey rock locally known as the "half-yard rock," above which we reach about five yards of fire-clay, of which I wish especially to remark that it contains numerous rounded balls of limestone, which are imbedded throughout it.

This thick clay is surmounted by a group of coals, of which the following is a section.

A noticeable feature in this coal, which is worked and is now known as the "Morlas Main," is the occurrence (rather too often) of "brasses" which are exclusively the pyritized stems of Calamodendron commune: many of these are very instructive specimens, which show the original structure of the wood as well as of the inner and outer bark.

For a reason which will be obvious presently, I wish to call especial attention to the almost unique mineral composition of these pyritized stems. Mr. D. Hesketh Richards, of Oswestry, gives the following- analysis of them:—