THE CHEMISTRY OF THE ROCKS. 697 



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, com- 

 pacted breccia so common among bowlders. It is as if the deeper 

 beds had slowly cooled under great compression and consequent im- 

 mobility 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 con- 

 gealed debris 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 vi- 

 cinity 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 ex- 

 posures or strongest attractions. Among the first of these cloud-pro- 

 ductions, 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 snow- 

 storms 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 cool- 

 ing vapors of the swollen sphere were struggling to unite and hold 

 fast the embrace against the antagonist force of heat ; when the con- 

 joined elements were pouring down their fiery torrents, and the air 

 was laden with the falling cinders and ashes of aerial 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 



