It may be argued that when the land stood at the higher level the range of the tides was less, and that consequently the amount of the proved subsidence may not be so great as 55 feet. The old land-surface on which the oaks grew lies, however, 35 feet below _mean tide_, so that any supposed lesser tidal range in ancient times could not make any great difference in the amount of subsidence here proved—it cannot be less than 45 feet. When, however, we notice the rapid increase in the range of the tides at the present day as the channel narrows towards Chepstow, and think what would be the probable effect of raising the whole country 50 or 60 feet, we are compelled to think that any narrowing and shoaling of the channel would have the effect of increasing, not decreasing, the tidal range at Barry Docks. In short it looks as if when the lowest submerged forest grew, the abnormal tides of Bristol may have extended further west, to near Cardiff. Whatever may have been the exact range of the tides in these early days, it seems that the Bristol Channel points to a subsidence in post-glacial times of about 60 feet—or just the same amount as the Thames, Humber, and Mersey. The amount may have been more; but the Barry Dock sections show that it cannot have been less; we will return to the question of its maximum extent later on. Before leaving this locality it may be well to enquire what further light it sheds on the movement of submergence, and on its continuous or intermittent character. The succession of the strata above the lowest land-surface, and the nature of their enclosed fossils, suggest long-continued but intermittent subsidence; I can see, however, no indication of a reversal of the process. The land-surface is carried beneath the water, the estuary then silts up, becomes fresh water, marsh-plants grow, and even trees may flourish on this marsh before it subsides again. But there is no sign that the strata were ever raised above the level to which ordinary floods could build up an alluvial flat. The land-surfaces seem always to have been swampy, and bed succeeds bed in fairly regular sequence, without the deep channelling we might expect to find when an alluvial flat was raised to a noticeable extent above the level of high water. The width of the Bristol Channel makes it clear that this gulf must occupy a submerged valley of great antiquity. It becomes therefore of interest to enquire whether the wide valley is correspondingly deep, or whether its rocky floor is found at the same shallow depth as in the case of the other river-valleys which we have been considering. The wide valley may have been formed in either of two ways. It may have been excavated as a deep valley with its bottom many hundred feet below the present sea-level. Or it may have commenced as the shallow valley of a big river with exceptionally powerful tides, and as this river swung from side to side it greatly widened its valley without making it any deeper.