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Precambrian rocks are at or near the surface in only about 10 percent of the conterminous United States, but it can reasonably be inferred that they comprise the continetal crust beneath about 90 percent. They are missing or unrecognized in the exotic terranes along the Pacific margin of North America, but they probably form significant parts of the crust in exotic terranes or continental fragments accreted to the eastern part of the continent during Paleozoic time. Thus, the total area of Precambrian rocks to be considered in this volume is comparable to that of the exposed Precambrian of the Canadian Shield. It is important to remember that in spite of the enormous lateral extent of the craton the volume of the continental crust is almost insignificant. The width of the North American craton is more than half the radius of the planet, but the thickness of the continental crust is less than one hundredth of the planetary radius (fig. 1). The volume of the crust is less than 2 percent of the volume of the mantle beneath the U.S. part of North America.

Precambrian rocks contain the only available record of the assembly and evolution of the fragile continental raft that we know as North America during more than four fifths of geologic time. Of the areas of exposed Precambrian rocks in the conterminous United States, about half have been covered by modern reconnaissance geologic mapping (scale 1:250,000 or larger); less than a quarter have been covered by detailed modern mapping (scale 1:62,500 or larger).

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