Copper occurs in traces, if not in determinable amounts, apparently, throughout the entire crust of the earth; but commercial deposits, although found in every type of rock, occur only where the crust has been fractured, fissured, or sheared. As the earth cooled to temperatures permitting chemical union of the atoms, the interaction of physical and chemical forces, notably gravity and the combination of the elements in the order of the heats of formation of their compounds, depressed the major parts of copper, sulphur, and other of the less chemically active heavy metals and their associated anions to a zone below the silicates where they remained until released through fractures in the solid crust.

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