here lie 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
