For the purpose of this article the continent of North America may be described as that portion of the lithosphere which lies between the Atlantic, Arctic, and Pacific oceans or between their suboceanic masses. Its area is that of the continental platform. Its depth may be taken at approximately 100 miles, the probable average limiting depth of the zone of isostatic adjustment according to Hayford.

This continental mass is of large dimensions and exhibits at its surface such variety of terranes and such diversity of geologic effects as to indicate beyond any reasonable doubt a heterogeneous constitution. Contrast the Canadian highlands with the Mississippi embayment, the Atlantic coastal plain with the Pacific Coast ranges, the New England metamorphic province with the Allegheny plateaus, the volcanic belt of the Cordillera with the non-volcanic regions of the East.

Heterogeneity may be a condition inherent to some extent in original constitution, . . .

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