formed, or possibly washing that again into sand and pebbles. This process continued through an indefinite period of time, forming layer upon layer of stratified rock, or excavating great hollows in the surface already formed. That period in the history of the earth’s crust before stratification began is known as Archean time. This was followed by the period known as Paleozoic time, which is divided into three ages. The first is the age of Invertebrates. It was during this age that life made its advent on the earth. The waters were the first to bring it forth, but before the close of the age it began also to appear on the land, in isolated spots, in the simplest forms of vegetation. The next age is known as the age of Fishes, during which vegetable life became more varied and abundant, winged insects floated in the air, and great sharks and gars swam in the seas. Then came the Carboniferous age or age of Coal Plants, in which vast areas of what are now the Middle, Southern, and Western States were covered with low marshes and shallow seas, and were rich and rank with multitudinous forms of vegetation. But these marshes were again and again submerged and covered with material washed up by the waves before the final subsidence of the waters left them as a continuing portion of the dry land. It was at the close of the Carboniferous age that great disturbances took place in the earth’s crust. Before this the rock strata had been comparatively level; now they were folded, flexed, broken, rounded into hills, pushed resistlessly up into mountain ranges. It was at this time that the upheaval of the great Appalachian Range in North America took place. Following this came Mesozoic time, which had but one age, the age of Reptiles. It was during this age that the type of reptiles reached its culmination. The land generally brought forth vegetation, though not with the prolific richness and luxury of the Carboniferous age. Birds, insects, and creeping things were abundant, and monsters of the saurian tribe swam in the seas, roamed through the marshes, crawled on the sandy shores, and took short flights through the air. The last great division is known as Cenozoic time, and covers two ages, the age of Mammals and the age of Man. It was during the mammalian age that trees of modern types, such as oak, maple, beech, etc., first made their appearance, and mammalian animals of great variety and size, both herbivorous and carnivorous, roamed through the forests. True birds flew in the air, true snakes crawled upon the ground, and in the waters were whales and many kinds of fishes of the present day. But the marine monsters and the gigantic and ferocious saurians of an earlier age had disappeared. So the world became fitted to be the dwelling-place of the human race. Then began the age of Man, an age which is yet not complete.