By far the best account that has come down to us of a Cornish tin stream-work carried below the sea-level, is that written by J. W. Colenso in 1829. Colenso had unusual opportunities for watching the works—apparently either as manager or owner—and he showed a most exceptional ability to note scientific points, such as were generally overlooked 90 years ago. It should be remembered that even in days before Lyell wrote we had in the Cornish tinners a class of men whose everyday occupations led them thoroughly to understand the action of running water. Their daily bread depended on their power to calculate where the ancient flood must have left the heavy tin-ore, where the barren ground would be found, or where old silted-up channels might be sought for. In their arrangements for diverting the streams in order to work the alluvial deposits, and for washing and concentrating the tin-ore, they were constantly brought face to face with the action of running water. When the buried tin-ground yielded anything abnormal the tinner recognised the effects of exceptional floods, of eddies behind boulders, or of obstructing ledges. Where he thought he saw the action of the deluge we may be pretty certain that he was dealing with something truly exceptional and outside his experience of the effects of a mountain torrent. He was not using the word as a cloak for ignorance or excuse for indifference, as was so often the case with the geologist of that day. Unfortunately most of the tinners could not write. Colenso’s account is entitled _A Description of Happy Union Tin Stream-work at Pentuan_. Pentuan lies at the mouth of the St Austell River, a rapid stream, much liable to sudden floods, which drains part of the granite and metalliferous region of St Austell Moor. The conditions are ideal for bringing down large quantities of the decayed granite which contains the tin-ore. This material was alternately weathered and broken up, and so sluiced with flood-water as to wash away the lighter quartz and felspar, thus concentrating the tin-ore, with a small amount of gold-dust and small gold nuggets, in the bottom layer. The alluvium of the St Austell River was therefore so profitable to work that every channel was followed upwards into the Moor, and the main valley was followed downward towards the sea. But as the coast was approached the rocky floor sank below the sea-level, so that this part was left till last, for it needed the diversion of the river and much pumping to get rid of the water. This, scientifically, is a fortunate circumstance, for of the earlier workings in the higher part of the valley no good accounts have come down to us.