126 Dr. Berger oji 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 particu- 

 larly hereafter. 



It is difficult to decide, whether the formation of this kaolin clay, 

 {xhQfcld-spath argiUforme^ of Haiiy) 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 arc 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 de- 

 composition 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.* 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 cVOrta in the Milanese, a grani- 

 tic 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 corncenne^ of 

 porphyries, of hornblende in mass, and of serpentine spontaneously resolve into clays, 

 fullers earth and maris: 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. Voy- 

 ages au Mont Perdu, p. 17. 



It is the carbonic acid according \.o Werner, which has changed the felspar into 

 kaolin in granite and gneiss, as well upon the walls of veins as upon the surface of moun> 

 tains. NouvellcTheorie de la formation des filons Journal dcs Mines, No. \s\\\. p. 84. 



