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 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 specific gravity, from common stones or pebbles; hence the name of “stream-works.” The nature of these deposits varies according to the positions which they occupy between the sea and the granite; whence the stanniferous strata were derived. Pentewan stream-work is one of the most interesting in the whole county. Its lowest bed consists of pebbles, gravel, and tin ore, and it rests on the solid rock. Immediately above this tin-ground is a black stratum of vegetable remains, among which are stumps of trees, standing erect, with their roots penetrating downwards into the bed of gravel. This subterranean forest stands forty-eight feet below high-water mark; showing that there must have been a change in the relative sea level. On this vegetable bed reposes a thick stratum of silt, intermixed with horns of deer, and with other relics of land animals, and also with detached pieces of timber. This silt is of the same description as that now forming in the Truro river, and in other estuaries on the coast; and it contains layers of shells peculiar to such situations. This silt is covered by a deep deposit of siliceous sand, in which occur various remains, principally of marine origin; and lastly, over this lies another bed of silt like the preceding, which reaches to the surface, where a thin marsh soil is now in a state of cultivation. The upper bed of silt is nearly on a level with the sea, being separated and protected from it by the interposition of a sandy beach. Many theoretical observations and reflections would naturally present themselves, after a statement of these facts; but such would be more appropriate to a separate treatise, than to a series of notices on individual parishes.