Everywhere this oldest part of the Western Hemisphere presents unmistakable signs of great age. The schists by their fine crumpling and scaly flakes of mineral show that they were formed deep in the bowels of the earth, for only there could they be subjected to the enormous pressure needed to transform their minerals into sheets as thin as paper. The coarse granites and gneisses proclaim still more clearly that they must have originated far down in the depths of the earth; their huge crystals of mica, quartz, hornblende, feldspar, and other minerals could never have been formed except under a blanket of rock which almost prevented the original magmas from cooling. The thousands or tens of thousands of feet of rock which once overlay the schists and still more the granites and gneisses must have been slowly removed by erosion, for there was no other way to get rid of them. This process must have taken tens of millions of years, and yet the whole work must have been practically completed a hundred or perhaps several hundred million years ago. We know this because the selfsame ancient eroded surface which is exposed in the Laurentian highland is found dipping down under the oldest known fossiliferous rocks. Traces of that primitive land surface are found over a large part of the American continent. Elsewhere they are usually buried under later strata laid down when the continent sank in part below sea-level. Only in Laurentia has the land remained steadily above the reach of the ocean throughout the millions of years.