such covering strata have been laid down. The fact that coal is sometimes found only in thin veins, with layers of shale and other material between, tells us that its origin must often have been a precarious affair, where alternate emergence and submergence would permit first the vegetation to develop and then its transformation to peat, followed by the deposition of fine sands or silt covering the bed. Several such cycles occurred, sometimes separated by untold ages of time, or again by much briefer periods. Certain mines, however, contain over 200 feet of solid coal. The length of time necessary for such a vast accumulation, or how many generations of these ancient plants went into their making, is beyond calculation. With the mining of coal running into the hundreds of millions of tons yearly, we get some idea of how great were those Carboniferous forests, and how extensive they were is proved from the widely separated localities in which coal mines are found. The Carboniferous age of fern, cycad, and conifer ancestors was by no means a quiet, orderly period, as from geological evidence it appears to have been much subject to alternate emergence and submergence of great tracts of land. Compared with what followed, it actually was a period of comparative quietness, however, and it must, in at least most parts of the world, have permitted the slow development of certain of its plant groups to a state of perfection never reached since. This is particularly true of the ancient relatives of our club mosses and horsetails.