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94 curve, and there cut a steep bluff, under which would be left the heaviest gravel. This observation and the noteworthy absence of any contemporaneous animal remains in the tin-ground, suggest that the bottom layer may date back to Pleistocene times, when the climate was colder and floods more violent.

It is not clear how far seaward the valley may then have extended; probably not more than half a mile at most. The tin ground was worked near Pentuan for 1400 yards along the valley, and averaged about 52 yards in breadth. So here again we meet with a fairly wide flat-bottomed valley, not a narrow V-shaped gorge; we may therefore take it that the base-level had been reached and that this base-level was identical with that met with in the rivers which open into Plymouth Sound.

(b) On the tin ground were rooted numerous oaks, which had grown and fallen on the spot. Their timber was so sound that Colenso applied one of the trees to make the axle of a water-wheel, and his comment on this is excellent. "It appears to me likely that at this period, the rising of the sea had so far checked the current of the river as to prevent its discharging the mud and sand brought down with it; thus the roots were buried [submerged?] to a considerable depth, and the trees killed, before the timber underwent its natural process of decay." At one spot he records finding oysters still remaining fastened to