kinds, and large predatory fish, swarmed in these old waters, and their calcareous shells and skeletons are now broken up by the quarryman and the collier as hard compact limestone. After these animals had lived and died in successive generations, perhaps for thousands of years, the downward movement of the earth's crust seems to have ceased for a while or to have become greatly less. The effect of this would be just to reverse what had been previously done, especially if a slight elevatory movement took place. The streams would in such circumstances descend from the uplifted ground with renewed velocity and transport their detritus to gradually increasing distances. The muddy and sandy sediment thus borne seawards would slowly silt over the coral-banks at the bottom, and in conditions so ungenial the organisms would dwindle down and finally die out. A great thickness of sand and mud would be spread out over their remains so long as the currents from the land continued to carry sediment out to sea, and thus probably originated the sandstones and shales superposed above the lowest limestone. Eventually the old steady downward movement returned, and with it the corals and stone-lilies. The detritus again sank to the bottom much nearer the land, forming great banks and shoals that choked up the river-mouth. Seaward the water regained its purity, and the bottom once more swarmed with living things. Another lapse of many thousand years may have here intervened during which the marine exuviæ gathered into another seam of limestone, until again the process of subsidence either ceased for a time, or what is perhaps more probable, became considerably feebler. Detrital matter began to creep seaward as before, and eventually entombed the corallines and crinoids to a great depth. The calcareous bed thus formed is the second limestone, and the superincumbent silt-beds represent the sandstones and shales that rest above it.