gram,[1] and I must once more repeat that this is no mere hypothesis, since the mass of sand and gravel cannot have been produced without an immense removal of the chalk.
Far, therefore, from requiring an immense flood of water, two hundred feet in depth, the accumulation of the gravel may have been effected by an annual volume of water, differing little from that of the present river.
A given quantity of water will, however, produce very different effects, according to the manner in which it passes. "We learn from observation, that a velocity of three inches per second at the bottom will just begin to work upon fine clay fit for pottery, and however firm and compact it may be, it will tear it up. Yet no beds are more stable than clay when the velocities do not exceed this: for the water even takes away the impalpable particles of the superficial clay, leaving the particles of sand sticking by their lower half in the rest of the clay, which they now protect, making a very permanent bottom, if the stream does not bring down gravel or coarse sand, which will rub off this very thin crust, and allow another layer to be worn off. A velocity of six inches will lift fine sand, eight inches will lift sand as coarse as linseed, twelve inches will sweep along fine gravel, twenty-four inches will roll along rounded pebbles an inch diameter, and it requires three feet per second at the bottom to sweep along shivery angular stones of the size of an egg."[2]
If, therefore, we are justified in assuming a colder climate than that now existing, we should much increase the erosive action of the river, not only because the rains would fall on a frozen surface, but because the rainfall of the winter months would accumulate on the high grounds in the form of ice and snow, and would every spring produce floods much greater than any which now occur.
We now come to the light-coloured sandy marl (Fig. 2). It is described by Mr. Prestwich as follows, "White siliceous sand and light-coloured marl, mixed with fine chalk grit, a few large sub- angular flints, and an occasional sandstone block, irregular patches of flint gravel, bedding waved and contorted, here and there layers with diagonal seams, a few ochreous bands, portions concreted. Sand and freshwater shells common, some mammalian remains."
In the pits at Amiens this bed is generally distinct from the underlying gravels, owing perhaps to the upper portion of the gravel having been removed; but in several places (Précy, Ivry, Bicêtre, &C.) we saw this section complete, the gravel coarser below becoming finer and finer, and at length passing above into siliceous sand. These sections evidently indicate a loss of power in the water at these particular spots, rapid enough at first to bring down large pebbles, its force became less and less until at length it was only able to deposit fine sand. This, therefore, appears to indicate a change in the course
