Page:A Treatise on Geology, volume 1.djvu/188

 172 abound in the upper parts of the carboniferous system, where coal abounds; they also occur in the midst of the millstone grits, and in sandstones and shales among limestones, especially where coal beds also are found; but they are almost unknown in the midst of the undivided limestone, where, however, a few algæ occur. In the true coal formation they are often accompanied by estuary, if not fresh water, shells, (unionidæ); but in Coalbrook Dale marine shells (such as lie in the mountain limestone) take their place. The large trees are sometimes found upright above the coal, while below it spread what seem to be their roots. Much of the substance of coal contains the structure of coniferous trees.

The suppositions which best connect the whole of the phenomena are, that the plants grew in and around swampy tracts by estuaries; that the land was sinking continually or by intermission; that thus the vegetable matter was gradually accumulated at and near the place of growth, and gradually or by intermission covered by marine estuary or river sediments.

The zoophyta of the carboniferous system are almost (perhaps wholly) absent from the coal formation: they are almost confined to the mountain limestone formation and to its calcareous portions, thus offering us most clear proof of the marine origin of that rock. When to this we add the absence of land reliquiæ from these limestones, it is evident that the materials of which these rocks are formed were not swept from the land like the substance of the arenaceous rocks, but elaborated from the salts of lime diffused in sea water. The zoophyta are partly of families almost extinct, as crinoidea; and partly of tribes yet abundant in the sea, as lamelliferous corals: the genera of corals often but not always (e.g. astræa, lithodendron) differ from those now living. The following summary is extracted from the 'Geology of Yorkshire,' vol. ii. p. 241.:—