Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/352

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Mr. William Phillips on the Oxyd of Tin.

some years ago, under the name of pea tin, perhaps from their size. They have evidently been rounded by attrition, and appear to be a species of wood tin from the variation in colour on the surface, which is generally of a hair-brown.

There is in my collection, a specimen, which I took from the heaps of tin on Poldice mine, that is of remarkable character. The general mass is of a light brown colour; minute veins of different shades of brown and black tin alternate in bands in the same direction. It is compact and hard, but not brittle, and gives sparks with a steel; its fracture is uneven. It may be well supposed from its great weight, to consist almost wholly of the oxyd of tin. It is accompanied by the black oxyd on one side, and very minute veins, apparently of quartz, traverse it in various directions.

Alluvial depositions of tin of considerable extent and depth have been found in several parts of Cornwall, which it is believed, is the only part of Europe in which tin occurs under these circumstances. The grains of it, which it may be presumed, are for the most part crystals rounded by attrition, are mostly very small, and sometimes exhibit marks of crystallization, generally of the macle. Stream tin affords from 65 to 75 per cent. of the purest grain tin. Its freedom from arsenic perhaps arises from the ore collected in the stream works being detached portions of the pure oxyd. And its presence in the regulus of tin procured from the ore of the veins may be supposed to arise from its being frequently accompanied by arsenicated iron. It is somewhat remarkable that the only traces of gold to be found in Cornwall, are in the stream works, in which it sometimes occurs in small grains, mostly detached, but occasionally accompanied by quartz. A few years ago, a specimen of considerable size was discovered, I believe in Carnan stream