Stephen's Church, and which is about six hundred and thirty-five feet above the level of the sea.
From Step-aside, we enter upon a mountain-plain of decomposed granite in the state of kaolin, which is famous for the porcelain earth it affords, and which is sent to Worcestershire. This plain is some miles in extent, and belongs to the southern boundary of the chain. One of the most elevated points of it, and which is in the neighbourhood of the principal quarry of the porcelain earth (China pit) is eight hundred and thirty feet above the level of the sea. This granite, the felspar of which forms two-thirds of the mass, appeared to me to be less decomposed near the borders of the plain, than in the central part. In this last place it has rather the appearance of a porphyry with a pulverulent base, of a whitish colour, in which crystals of quartz, and some plates of mica, are loosely included. It is used in this rough state in the manufacture of porcelain, in the same manner as the Chinese make use of petuntze, by mixing it in certain proportions with the porcelain earth that is obtained by washing and frequent precipitations. Crystals of a compact and earthy felspar are occasionally met with in this decomposed granite, of a much larger size than usual. We also find here another rock, the geological position of which is very interesting; I mean the schorl rock,[1] a binary compound of schorl and quartz, in which the first considerably predominates. We find masses of it of all sizes, generally of a rounded shape, and coated on the surface with the porcelain earth. There are also large blocks of it in the bed of the Fal,
- ↑ “ The schorl rock of Cornwall is probably very intimately connected with topaz rock.” Jameson's Elements of Geognosy,
The same author adds in a note “ The geognostic relations and characters of schorl rock are not well ascertained., therefore I have declined saying any thing regarding it in the text.”
