our present atmosphere, would wrap up the earth in a darkness gross and palpable as that of Egypt of old,--a darkness through which even a single ray of light would fail to penetrate. And beneath this thick canopy the unseen deep would literally "boil as a pot," wildly tempested from below; while from time to time more deeply seated convulsion would upheave sudden to the surface vast tracts of semi-molten rock, soon again to disappear, and from which waves of bulk enormous would roll outwards, to meet in wild conflict with the giant waves of other convulsions, or return to hiss and sputter against the intensely heated and fast foundering mass, whose violent upheaval had first elevated and sent them abroad. Such would be the probable state of things during the times of the earlier gneiss and mica schist deposits,--times buried deep in that chaotic night or "evening" which must have continued to exist for mayhap many ages after that beginning of things in which God created the heavens and the earth, and which preceded the first day. To a human eye stationed within the cloud, all, as I have said, must have been thick darkness: to eyes Divine, that could have looked through the enveloping haze, the appearance would have been that described by Milton, as seen by angel and archangel at the beginning of creation, when from the gates of heaven they looked down upon chaos:-- "On heavenly ground they stood, and from the shore They viewed the vast immeasurable abyss, Outrageous as a sea, dark, wasteful, wild, Up from the bottom turned by furious _heat_ And surging waves, as mountains to assault Heaven's height, and with the centre mix the pole." At length, however, as the earth's surface gradually cooled down, and the enveloping waters sunk to a lower temperature,--let us suppose, during the latter times of the mica schist, and the earlier times of the clay slate,--the steam atmosphere would become less dense and thick, and at length the rays of the sun would struggle through, at first doubtfully and diffused, forming a faint twilight, but gradually strengthening as the latter ages of the slate formation passed away, until, at the close of the great primary period, day and night,--the one still dim and gray, the other wrapped in a pall of thickest darkness,--would succeed each other as now, as the earth revolved on its axis, and the unseen luminary rose high over the cloud in the east, or sunk in the west beneath the undefined and murky horizon. And here again the _optical_ appearance would be exactly that described by Milton:--