The Cornish rivers yield most valuable information. It so happens that many of them bring down large quantities of tin ore from the granitic regions, and this ore being very heavy tends to find its way to the bottom of the alluvial deposits, out of which it is obtained in the same way as alluvial gold in other countries. On following this detrital tin ore downwards towards the estuaries the “tinners” or alluvial miners found in many cases that a rich layer descended lower and lower till it passed well below the sea-level in some of the ancient silted-up valleys. Some of these tin deposits were so rich that it paid even to divert the rivers, dam out the sea, and remove the alluvium to considerable depths in search of the ore. These excavations for tin produced most interesting and continuous sections of the alluvial deposits, and if only they had been examined more thoroughly and scientifically they would have thrown much light on the questions we are here considering. Unfortunately all the deeper excavations were made in days when all ideas as to the origin of “diluvial” deposits were so tinged with theories as to the effects of a universal deluge, that many of the most interesting points escaped notice. The last of these “stream works” was closed many years ago. Notwithstanding the early date of these excavations, some most interesting observations were recorded; and though they make us long for fuller details and regret the loss of many of the objects referred to, we must be grateful that so much was noted, and by such careful observers. This entire removal of the old alluvial deposits—for the tin usually occurs concentrated in the bottom layers—showed clearly that in Cornwall, as elsewhere, old land-surfaces can be found far below the sea-level. The shape of the valley-bottom, and the rapid lessening of its fall as the coast is approached, in several cases point clearly to the proximity of the sea, and show that its ancient level must have been about 70 feet below present tides. Here it may be pointed out that as the sea-level is approached the steady seaward fall of a rocky =V=-shaped valley quickly lessens, changes to a gentle slope, and then to a flat, more or less wide according to the length of time during which the river has been kept at the same level, and could only swing from side to side, without deepening its bed. In Cornwall there is a definite limit below which the erosion of the valleys has not gone, and at this level the valley widens and flattens as it does elsewhere.