For the intervening or fourth day we have that wide space represented by the Permian and Triassic periods, which, less conspicuous in their floras than the period that went immediately before, and less conspicuous in their faunas than the periods that came immediately after, were marked by the decline, and ultimate extinction, of the Palæozoic forms, and the first partially developed beginnings of the Secondary ones. And for the first and second days there remain the great Azoic period, during which the immensely developed gneisses, mica schists, and primary clay slates, were deposited, and the two extended periods represented by the Silurian and Old Red Sandstone systems. These, taken together, exhaust the geologic scale, and may be named in their order as, _first_, the Azoic day or period; _second_, the Silurian and Old Red Sandstone day or period; _third_, the Carboniferous day or period; _fourth_, the Permian and Triassic day or period; _fifth_, the Oolitic and Cretaceous day or period; and _sixth_, the Tertiary day or period. Let us attempt conceiving how they might have appeared pictorially, if revealed in a series of visions to Moses, as the successive scenes of a great air-drawn panorama. During the Azoic period, ere life appears to have begun on our planet, the temperature of the earth's crust seems to have been so high, that the strata, at first deposited apparently in water, passed into a semi-fluid state, became strangely waved and contorted, and assumed in its composition a highly crystalline character. Such is peculiarly the case with the fundamental or gneiss deposits of the period. In the overlying mica schist there is still much of contortion and disturbance; whereas the clay slate, which lies over all, gives evidence, in its more mechanical texture, and the regularity of its strata, that a gradual refrigeration of the general mass had been taking place, and that the close of the Azoic period was comparatively quiet and cool. Let us suppose that during the earlier part of this period of excessive heat the waters of the ocean had stood at the boiling point even at the surface, and much higher in the profounder depths, and further, that the half-molten crust of the earth, stretched out over a molten abyss, was so thin that it could not support, save for a short time, after some convulsion, even a small island above the sea level. What, in such circumstances, would be the aspect of the scene, optically exhibited from some point in space elevated a few hundred yards over the sea? It would be simply a blank, in which the intensest glow of fire would fail to be seen at a few yards' distance. An inconsiderable escape of steam from the safety-valve of a railway engine forms so thick a screen, that, as it lingers for a moment, in the passing, opposite the carriage windows, the passengers fail to discern through it the landscape beyond. A continuous stratum of steam, then, that attained to the height of even