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.

Today this old, old land might be as rich as many others if
climate had been kind to it. Its soil, to be sure, would in many
parts be sandy because of the large amount of quartz in the
rocks. That would be a small handicap, however, provided the soil
were scores of feet deep like the red soil of the corresponding
highland in the Guiana region of South America. But today the