If the original granite is the father of the rocks, the sea is the mother. In her womb they were gestated and formed. Had not this seesaw of land and ocean taken place, there could have been no continental growth. Every time the land took a bath in the sea, it came up enriched and augmented. Each new layer of rocky strata taken on showed a marked change in color and texture. It was a kind of evolution from that which preceded it. Whether the land always went down, or whether the sea at times came up, by reason of some disturbance of the ocean floors in its abysmal depths, we have no means of knowing. In any case, most of the land has taken a sea bath many times, not all taking the plunge at the same time, but different parts going down in successive geologic ages. The original granite upheavals in British America, and in New York and New England, seem never to have taken this plunge, except an area about Lake Superior which geologists say has gone down four or five times. The Laurentian and Adirondack ranges have never been in pickle in the sea since they first saw the light. In most other parts of the continent, the seesaw between the sea and the land has gone on steadily from the first, and has been the chief means of the upbuilding of the land. To the slow and oft-repeated labor-throes of the sea we owe the continents. But the sea devours her children. Large areas, probably continental in extent, have gone down and have not yet come up, if they ever will. The great Mississippi Valley was under water and above water time after time during the Palaeozoic period. The last great invasion of the land by the sea, and probably the greatest of all, seems to have been in Cretaceous times, at the end of the Mesozoic period. There were many minor invasions during Tertiary times, but none on so large a scale as this Cretaceous invasion. At this time a large part of North and South America, and of Europe, and parts of Asia and Australia went under the ocean. It was as if the earth had exhaled her breath and let her abdomen fall. The sea united the Gulf of Mexico with the Arctic Ocean, and covered the Prairie and the Gulf States and came up over New Jersey to the foot of the Archaean Highlands. This great marine inundation probably took place several million years ago. It was this visitation of the sea that added the vast chalk beds to England and France. In parts of this country limestone beds five or six thousand feet thick were laid down, as well as extensive chalk beds. The earth seems to have taken another hitch in her girdle during this era. As the land went down, the mountains came up. Most of the great Western mountain-chains were formed during this movement, and the mountains of Mexico were pushed up. The Alps were still under the sea, but the Sierra and the Alleghanies were again lifted.