Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/138

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126

Dr. Berger on the physical Structure

near St. Stephen's Church. This is not the only place in Cornwall where I found that scarce rock, of which I shall speak more particularly hereafter.

It is difficult to decide, whether the formation of this kaolin clay, (the feld-spath argiliforme, of Haüy) is connected with a particular texture of the felspar, dependent on some principle which is inherent in it in the places where this earth is met with; or whether, as I should be more inclined to believe, we are to attribute it solely to the action of external agents, particularly of the water retained in the crevices of the native rock of the place, which produces a decomposition in one portion of the rock, then acts upon the adjacent parts, and so by degrees, in time extends its effects to a considerable distance.^[1] Whatever be the cause, we know that kaolin is never found but in a primitive country, and forming beds or veins in granite, particularly in that species called graphic granite.

To the already pretty extensive enumeration of the places where kaolin is met with, such as China, Japan, different parts of Germany, of France, &c. I shall add another, which as far as I know has not yet been mentioned, viz. the Culma d'Orta in the Milanese, a granitic mountain, elevated one thousand four hundred and fifty-eight

↑ Ramond found granites in the high chain of the Pyrenees, corroded both externally and internally: not detached blocks alone, he informs us, but whole regions are attacked with this cariousness, the cause of which is still unknown. This corrosion is frequently met with on the northern confines of the chain, where beds of cornéenne, of porphyres, of hornblende in mass, and of serpentine spontaneously resolve into clays, fullers earth and marls: these still preserve the appearance and grain of the rock which has produced them, though they now only form an earth easily cut by the knife. Voyages au Mont Perdu, p. 17.
It is the carbonic acid according to Werner, which has changed the felspar into kaolin in granite and gneiss, as well upon the walls of veins as upon the surface of mountains. Nouvelle Théorie de la formation des filons. Journal des Mines, No. xviii. p. 84.

[1] Ramond found granites in the high chain of the Pyrenees, corroded both externally and internally: not detached blocks alone, he informs us, but whole regions are attacked with this cariousness, the cause of which is still unknown. This corrosion is frequently met with on the northern confines of the chain, where beds of cornéenne, of porphyres, of hornblende in mass, and of serpentine spontaneously resolve into clays, fullers earth and marls: these still preserve the appearance and grain of the rock which has produced them, though they now only form an earth easily cut by the knife. Voyages au Mont Perdu, p. 17.
It is the carbonic acid according to Werner, which has changed the felspar into kaolin in granite and gneiss, as well upon the walls of veins as upon the surface of mountains. Nouvelle Théorie de la formation des filons. Journal des Mines, No. xviii. p. 84.

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