Page:The parochial history of Cornwall.djvu/92

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50
ST. AUSTELL

will be given hereafter, in the accounts of St. Stephen's, and of the adjacent parishes. Carclaze tin mine must not, however, be passed by, as it is one of the greatest curiosities in Cornwall. This mine is worked "open to the day" (according to the miners' term), that is, like a quarry. It is of a considerable depth; and its superficies exceeds several acres in extent. It is excavated entirely in a white granite, somewhat similar to the disintegrating variety above alluded to; and when the sun shines, the reflection of light is so exceedingly dazzling as to be almost insupportable. The tin ore occurs here intermixed with shorl and quartz, in the form of short irregular veins, which traverse the granite in every direction, and so abundant, that the whole rock requires to be pounded and washed to complete an entire separation of the ore.

Hornblend rocks succeed the granite, and produce a red fertile soil. These extend a little to the south of the town of St. Austell, and are followed by a blue lamelar slate, in which the mines are situated. This rock is much softer, and more argillaceous than the hornblend slate, and decomposes into a light-coloured soil. The matrix of its lodes abound in chlorite: it is probably a a chlorite schist. This formation is traversed by several beds of felspar, porphyry (elvan courses), in the western side of the parish, which run north-east and south-west, in a somewhat tortuous manner, and dip towards the granite. One of these elvans, near Pentewan, has been extensively quarried, and is much esteemed as a building material. This chlorite slate also contains, in the cove at Duporth, a bed of compact magnesian rock, abounding in asbestos; and passes on either hand into the surrounding slate, by means of layers of talcous schist.

This parish has long been celebrated for its stream works, which are diluvial beds containing tin ore. They are generally found in deep vallies where rivulets flow, which are used in separating the tin ore, by its inferior