. The Edinburgh new philosophical journal . 296 On the Physical Geography of the Fig. 7. Fig. 7 shows the direction of the es- tuary at the period of the fluvio-marine strata, with the line of the rise and fall of the tide, and the shoal in its altered position. The change from the first to the latter condition was not sudden, as is proved by the directions of the cur- rents in the intermediate beds being intermediate, scribed above ; but yet it does not necessarily follow that it was uniformly slow and gradual. These facts, and the ver- ticality of the strata in Alum and Whitecliff Bays, rend
. The Edinburgh new philosophical journal . 296 On the Physical Geography of the Fig. 7. Fig. 7 shows the direction of the es- tuary at the period of the fluvio-marine strata, with the line of the rise and fall of the tide, and the shoal in its altered position. The change from the first to the latter condition was not sudden, as is proved by the directions of the cur- rents in the intermediate beds being intermediate, scribed above ; but yet it does not necessarily follow that it was uniformly slow and gradual. These facts, and the ver- ticality of the strata in Alum and Whitecliff Bays, render it difficult to ascertain the exact period of the change. In the vicinity of Barton it appears to have occurred, so that the second line of axis was fully established when the strata just above the Barton Clay were deposited; whereas in Alum Bay this was not the case until the period of the upper portion of the Headon Hill marls and limestones ; as though, like the passage from marine to freshwater conditions, the change had progressed from the west towards the east. "When completely formed, the general mean line of the axis of the estuary does not appear to have varied much ; though its configuration may nevertheless have changed very materially; its depth, and the position of its bounding coasts, may have undergone great al- terations. Indeed, the alternation of beds of different physical character, independent of what is learned from the study of the organic remains, in my opinion clearly indicates that such changes did occur; but these might have been very consider- able, and yet the mean line of axis might have remained near- ly the same. For instance, in the case of the present estuary of the Thames, if, by subsidence or other changes, it was to be extended, so as to comprise the whole of the London Tertiary basin up to the range of the Chalk, so as to cover the whole of the triangle included between Norwich, Hungerford, and Can- terbury, we should have a very gre
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