INDUSTRIES This is the origin of the ' stream tin ' deposits in the valleys of Cornwall, especially those to the south of the watershed, and, to a lesser extent, in the valleys of the Dartmoor rivers. Probably attention was first directed to deposits of stream tin by an agency similar to that lead- ing to their formation. Streams and rivers, swollen by rains, would cut deeper gutters through the alluvium of their valleys, and expose layers of tin stones, pebbles, and gravel. What was thus shown to occur in several valleys would be anticipated and sought in similar situations elsewhere, although the surface indications might not precisely correspond or be so decisive ; and, by degrees, discovery would become an art. Nor could stream works be long in operation without some evidences of their connexion with the lodes in adjacent hills. The early miners might not recognize the fact that the quantity of tin stone washed down into the valleys and moors was a measure of the denudation of the more elevated regions of the country; but they could not fail, as they worked upward, to dis- cover some traces of the veins from which stream tin had been derived. Hence, unquestionably, arose the practice of ' shoding.' J 'The ores of tin,' wrote Pryce in 1778," ' are shode, stream, and mine. The shode is adjacent to and scattered to some distance from its parent lode, and consists of pebbly and smoothly angular stones of various sizes, from a half-6unce to some pounds in weight. Stream tin is the same as shode, but smaller in size and arenaceous, and in that state is formed of small pyramids of various planes, broad at the base and tapering to a point at the top. Stream tin ore is the smaller loose particles of the mineral de- tached from the bryle, or backs of sundry lodes, situated on hilly ground, and carried down into the vales by the retiring waters of the floods. In the solid rock of the valley there is no tin ore, but immediately upon it is deposited a layer of stream tin of various thicknesses, perhaps over that a layer of earth, clay, or gravel, and upon that another stratum of tin ore, and so on suc- cessively, stratum on stratum, according to their gravity, and the different periods of their coming. Mine ore,' he goes on to say, ' is the original lode, buried usually in rocky substances in the hills or the cliffs.' We cannot end this description of the tin beds, so essential to the proper understanding of the history of Cornish mining, better than by an account of an old stream work discovered about a century ago, and mentioned by the historian Polwhele. 'They (the Forth stream works) were situated near the shore of Trewardreth Bay ; the ore was of the purest kind, and con- tained two-thirds metal. The pebbles from which 1 Worth, Historical Notes concerning the Progress of Mining Skill, 5. 1 Pryce, Minerahgia Cornubiensis, 66. the metal was extracted were in size from sand- like grains to that of a small egg. They were included in a bluish marl, mixed with sand and containing various marine excuviae. The depth of the principal bed was nearly twenty feet, and its breadth six or seven. This appears to have been worked at a very remote period, and before iron tools were employed, as large pickaxes of oak, holm, and box, have been found there. In St. Blazey, St. Austell, St. Stephen in Bran- nel, and St. Ewe, are many old stream works which men commonly attributed to Jews. The most considerable stream of tin in Cornwall is that of St. Austell Moor, which is a narrow valley about a furlong wide (in some places some- what wider) running nearly three miles from the town of St. Austell southward to the sea. On each side, and at the head, above St. Austell, are many hills, betwixt which are little valleys, which all discharge their waters and whatever else they receive from the higher grounds into St. Austell Moor, whence it happens that the ground of this moor is adventitious for about three fathoms deep, the shodes and streams from the hills on each side being here collected and caught into floors according to their weight and the suc- cessive dates of their coming thither. The uppermost mat consists of thin layers of earth, clay, and pebbly gravel, about five feet deep. The next stratum is about six feet deep, more stony, the stones pebbly formed, and with a gravelly sand intermixed. These two coverings being removed they find great numbers of tin stones from the bigness of a goose-egg, and larger, down to the size of the finest sand. The tin is in- serted in a stratum of loose, smooth stones, from a foot diameter down to the smallest pebbles. From the present surface of the ground to the solid rock or " karn " is eighteen feet deep at a medium. This stream tin is of the purest kind, and a great part of it, without any other manage- ment than being washed on the spot, brings thirteen parts for twenty at the melting-house.' 3 From the shallowness of the stream - tin deposits and the comparative ease with which they could be shovelled out, as contrasted with the difficulties of driving shafts through the rock, it goes without saying that of the two methods the former was the first to be employed. All discoveries of ancient tin mines have been made in diluvial ground, 4 and it may be stated with some degree of certainty that stream tinning prevailed in the early and the mediaeval periods, to the exclusion of lode mining, save possibly when the latter was carried on upon remarkably rich lodes and in shallow depths. 5 A few facts may be cited in support of this statement. The composition
- Polwhele, Hist, of Cornw. bk. z, p. 10.
- For an example of this see ' Description of a Stream
Work at Drift Moor, near Penzance,' by Jos. Came, Trans. Roy. Geol. Soc. Cornw. iv, 47-56. 5 Polwhele, Hist, of Cornw. i, Supplement, 64. 541
