If the earth in the beginning cooled down from a fluid state, by the time a solid crust had formed, gravity, diffusion, and the order of chemical combination in a cooling system, would have differentiated and segregated silicates and the heavy metal ores in zones, or layers, so widely separated that the small volume of magma entering a magma chamber in the outer crust of the earth would not be likely to contain, except in traces, the great variety of disparate elements found in an ore-bearing terrane.Since practically all deposits of primary ores of the heavy metals have entered the host rock through fractures, shears, fissures, or open pores, existent in the rock prior to the introduction of the ore-bearing fluid, and since the host rock is more often sedimentary or metamorphic than igneous, it is logical to conclude that the relation between ore deposits and igneous intrusives is structural rather than genetic.

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