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Page:Popular Science Monthly Volume 4.djvu/717

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THE CHEMISTRY OF THE ROCKS.

It is as if chances as variable as winds and storms had regulated the production and mixture. There is every gradation in the texture of granite, from the fine-grained blocks of the quarry to the coarse, compacted breccia so common among bowlders. It is as if the deeper beds had slowly cooled under great compression and consequent immobility of the particles, while the superficial layers had been worked up and conglomerated at the surface. There are specimens of granite composed of massive angular crystals, that seem as if they had been thrown together and cemented. It is, again, as if they were the congealed débris of some terrific hail-storm of quartz, mica, and feldspar.

After the greater part of the silicious minerals had been deposited, and the cooler exterior gases had thus been let down to a nearer vicinity with the heavier vapors, we find that the metals proper began gradually to condense and fall. Those which have no active affinities for the other elements were deposited in their native purity. Others took on the forms of oxides or sulphurets, according to their first exposures or strongest attractions. Among the first of these cloud-productions, the rock records tell us, were the scanty rainfalls of gold and platinum, and the more plentiful showers of silver and copper. Rivulets of native ores ran along the hardening crust, filling the veins and crevices, or mingling with the liquid quartz that was seaming the granite and gneiss.

Then from clouds of condensing iron vapor, that must have burned and scintillated with indescribable magnificence, fell the thick heavy storms of the black lodestone, the blood-red hematite, or the dark-yellow pyrites. Possibly storm-centres were established, over which the cyclones were held concentrated, and often repeated by force of intense magnetic attractions which have left their traces in almost every iron-mine.

Following these, at times and places, came on the great snowstorms of the waxy flakes of zinc-blende, and the pearly calamine, the red oxide or the white carbonate of lead, and the gray galena, the beautiful crystals of the tin-stone, the gray plumes of antimony, and all the tinted and varied forms of the less abundant ores and alloys. Meanwhile, through all the long ages of these metallic precipitations, there was continually falling over all the earth the white, impalpable powder of lime—the element calcium condensed into cloud-mist, and oxidized in the upper regions of the air.

These were the great chemical periods of our world; when the cooling vapors of the swollen sphere were struggling to unite and hold fast the embrace against the antagonist force of heat; when the conjoined elements were pouring down their fiery torrents, and the air was laden with the falling cinders and ashes of aërial conflagrations; when the vast workshop of Nature was forming and sorting its raw materials.

We do not, however, wish to be understood as insisting that all