Page:Popular Science Monthly Volume 72.djvu/538

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T is well understood, but often forgotten, that all the constituents of ore deposits are found in some form in the earth's crust, contained in more or less abundance in the rocks, especially in the eruptive rocks; and that they have been in some way collected from their disseminated condition in these rocks, and concentrated in veins, beds or other deposits.

Analyses of fresh eruptive rocks have demonstrated the existence therein of all of the ingredients of our valuable ores and their compounds. Few of them occur native like gold, silver, copper and platinum; and often, because of their minute quantity and fine state of subdivision, it is not possible to determine the precise form in which they are present.

The presence of sulphur, arsenic, antimony and tellurium indicates that there may be many metallic combinations in the eruptive magmas similar to those formed at later periods, nearer the surface.

The average composition of the earth's crust has been approximately estimated as follows: