274 EXPERIMENTS ON METAMORPHISM AND 



in Brazil, and in the Alleghanies,* that these formations, at the same 

 time gold-bearing and metamorphic, attain great dimensions. In 

 short, the metalliferous deposits, as well as the silicious discharges 

 which furrow many countries, are but peculiarities of metamorphic 

 phenomena. 



CHAPTER VI. 

 DECOMPOSITION OF SILICIOUS ROCKS, AND MINERAL SPRINGS. 



Causes, -which appear to have some analogy with those which have 

 changed the sedimentary deposits into crystalline rocks, have often 

 produced an inverse effect, and have transformed silicious crystalline 

 rocks into hydrated silicates; sometimes earthy and amorphous, such 

 as clays, steatite, seladonite; sometimes crystalline, as the zeolites. 

 We know, especially since the researches of Ebelmen, how atmo- 

 spheric agents gradually decompose the silicious Tocks.f The carbonic 

 acid of the atmosphere, the nitric acid which is there daily developed, 

 as the very general phenomenon of nitrification proves — in fine, the 

 organic acids resulting from the decomposition of vegetable matter, 

 gradually dissolve out the alkalies and the alkaline earths, and the 

 residues, in which alumina is more and more concentrated, finally be- 

 come hydrosilicates of alumina of the family of clays. But it is not 

 alone in the neighborhood of the atinosphere that clays may have 

 been derived from the transformation of crystalline silicious rocks. 

 Volcanic fumaroles often reduce the masses through which they pass 

 into real clays, ordinarily variegated, (Lipari islands, Solfatares of 

 Pouzzoli, Iceland, Azores, Kamtschatka.):}: This is, without doubt, 

 the origin of the mud sometimes thrown out by the great volcanoes of 

 the Andes. Carbonic acid, as Fournet long ago discovered in Au- 

 verne, itself suffices to produce analogous decomposition. The alka- 

 line solutions which many hot springs contain, those of Plombieres, for 

 instance, appear to decompose the silicious rocks as effectually as the 

 acids of the fumaroles. 



Like effects are often found in the neighborhood of metalliferous 

 deposits. Thus the beds of kaolin, in Saxony, certainly result from 

 a decomposition of granite near veins of iron, which traverse it; this 

 example may be considered as the type of many facts of the same 

 kind.§ The various masses of kaolin which are worked in Cornwall 



~ A recent memoir of Leiber on this country gives interesting and detailed accounts. 



fl can only mention here the important labors of Fuchs, Berthier, Forchhammer, Tur- 

 ner, Fournet, Al. Brongniart, Malaguti, and others on this subject. 



J According to the observations of Breislack, Hoffman, Bunsen, D.arwin, and others. 

 Charles Devilln has explained by an experiment this reaction of sulphuretted hydrogen on 

 rocks. 



§ Sosa, in Saxnoy, near a vein of quartz containing iron ore ; environs of AIen9on, near 

 a vein of quartz ; deposits of the Loire, according to Gruner and Rozet ; decomposed 

 feldi^par of the arkose of central France, &c.; halloysite of Louhassoa, near Bayonne ; kaolin of 

 Eschassieres, (AUier,) in feldspathic porphyry, according to Boulanger. 



It was facts of this kind that induced Brongniart and Malaguti to attribute the decom- 

 position of feldspar to voltaic action. 



