Brongniart says, that “tin belongs exclusively to primitive countries, and even to those of the oldest formation; for not only is it found in veins and beds in granite, but also in masses, or disseminated in beds of gneiss, of micaceous shistus, and of porphyry. Veins of tin adhere very often to the walls of the lode, by the rock which encloses them; they are always divided by other veins, and never divide them. Tin therefore seems to be one of the metals of the oldest formation: it is accompanied by substances which belong to the same age, such as wolfram, arsenicated iron, topaz, quartz, fluated lime, phosphated lime, hornblende, green and black mica, chlorite, &c. whilst carbonated lime, sulphates barytes, zinc, lead and silver, substances which frequently accompany other metals, are rarely found with it.” All this may perhaps be true, in so far as it regards depositions of tin in other countries, but I am induced to believe it is not wholly so in regard to Cornwall, where veins producing tin often occur in districts, both granitic and schistose, which it seems difficult to ascribe to the primitive formation.
Dr. Berger in his paper on the physical structure of Devonshire and Cornwall, says, “Here (Cornwall), as in the Hartz, it (grauwacke) is very rich in ore.” In the term grauwacke, Dr. Berger, following the example of Werner, seems so include every species of that rock, usually called schist, and by the miner killas. He further says, “Grauwacke is one of the oldest of the secondary rocks.” Again,  in speaking of the discovery of uranium in Tin Croft mine, which is situated at the foot of a granitic hill, but partly in granite and partly in schist, its “being found in this district proven that, contrary to the opinion of Werner, it may be met with in secondary mountains.”
- Traité Elem. p. 191.
- Geolog. Trans. vol. I. p. 113.
- p. 111.
- p. 470.