200 ON THE CHINA-STONE AND CHINA-CLAYS OF CORNWALL, 



felspar, in large red or white opaque, cubic, or rhomboidal crystals ; while 

 on the south it is separated from the neighbouring granite by a large elvan 

 dyke ; and it is worthy of notice, that -while on one side of this, you 

 may find China-stone perfectly pure, on the other, only from one to two 

 feet distant, the stone is rendered useless by the presence of small plates 

 of talc embedded in dense grey granite, which also forms a portion of 

 the eastern boundary. 



Any one who has carefully studied the porphyry dykes, or the 

 general nature of the primary rocks, of Cornwall, cannot but have 

 noticed the difference in the temperature at which some of them have 

 been upheaved compared to that of others, for while some of our 

 granites are composed of substances which have in their crystals a cer- 

 tain amount of water that has not been lost, others have no trace of it, 

 their felspar having become an amorphous-looking powder (kaolin) ; and 

 others presenting the same waxy edge on fracture that is noticed in por- 

 celain, particularly the elvan dykes. From this it has been conjec- 

 tured, though to me it appears doubtful, that as the melting point ot 

 other minerals was considerably below that of these rocks, at the time 

 of the extraordinary convulsion to which our country has been subjected, 

 the China-stone was by this means freed from iron, &c. ; and that, on 

 its having reached the surface, the water by which it was surrounded at 

 once caused the crystals of felspar to split, lose their outline and charac- 

 ter, and become easily acted on by the solvent power of rain-water, 

 which by depriving it of a portion of its potash, leaves the crystals of 

 quartz or silicic acid and plates of mica, glistening with a silvery hue 

 imbedded in a mass of silicate of potash and alumina ; which, from the 

 loss of crystallization, cannot be termed felspar, nor is it kaolin, for it 

 has not been subjected sufficiently long to the causes which lead to its 

 formation. 



The chief causes which I believe to heve led to its disintegration, 

 and not only to the formation of China-stone, or China-clay, but to that 

 of all the land at present in cultivation or capable of being cultivated, 

 aie, 1st, — external physical agents, proved by the fact that China-stone 

 is very seldom found at a depth of more than from 20 to 30 feet from 

 the surface ; the influence of the seasons ; the changes from hot to cold 

 on a body composed of crystals possessing such different expansive 

 powers as those of felspar and quartz ; and the solvent power of rain- 

 water : while, as chemical agents, we have, 2ndly, — the influence of the 

 excess of carbonic acid in the air, as well as that from the interior of 

 the earth, of the influence of which we have abundant proof in the 

 excellent crops obtainable near lavas, or wherever this gas can gain ac- 

 cess to the compound silicates of which the greatest portion of the 

 Earth's crust consists ; and by the influence of respiration in rooms pro- 

 vided with windows, which may have been exposed for a long period to 

 its application. 



At present, while there is a great demand for the article, the spot 



