China-clays of Cornwall. 95 



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. 



Many have thought, and do still suppose, that the clay is 

 gradually forming into granite, and confidently assert that 

 the whole of the middle granite tract was undoubtedly formed 

 from clay beds ; the geologist, I need scarcely add, will be 

 able to estimate this at its proper worth : others also add, 

 that this mass has been thrown up in the water, which at first 

 covered it and fell back on itself, which they assert accounts 

 for the flattened outline the tops of the hills of this district 

 present. 



The chief causes which I believe to have led to its disin- 

 tegration, and not only to the formation of China-stone, or 

 China-clay, but to that of all the land at present in cultiva- 

 tion or capable of being cultivated, are — 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 1 

 such different expansive powers as those of felspar and quartz; 

 and the solvent power of rain-water : while, as chemical 

 agents, we have, 2dhj, 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 ex- 

 cellent crops obtainable near lavas, or wherever this gas can 

 gain access to the compound silicates of which the greatest 

 portion of the earth's crust consists ; and by the influence of 

 respiration in rooms provided 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 from whence China-stone is procured presents the 

 appearance of a large rabbit burrow, as there are no less 

 than nine sets for the district, the proprietor of each of which 

 has his portion of the hill covered with the mouths of pits, 

 around which are stationed a number of men with their 

 waggons, who, after the China-stone has been raised by quar- 

 rying and the employment of powder, carry it to one of the 



