262 ORIGINAL AETICLES. 



gram,* and I must once more repeat that this is no mere hypo- 

 thesis, since tlie mass of sand and gravel cannot have been produced 

 wdthout 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 jiarticles 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 ^^ermanent 

 " bottom, if the stream does not bring down gravel or coarse sand, 

 " which A^dll 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 peb- 

 " bles 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."t 



If, therefore, we are justified in assuming a colder . climate than 

 that now existing, we should much increase the erosive action of the 

 riA^er, not only because the rains would fall on a frozen surfiice, 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 

 prodiice floods much greater than any which now occur. 



We now come to the light-coloured sandy maid (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 un- 

 derlying gravels, owing perhaps to the upper portion of the gravel 

 having been removed ; but in several places (Precy, Ivry, Bicetre, 

 &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 par- 

 ticular spots, rapid enough at first to bring down large pebbles, its 

 force became less and less until at length it Avas only able to deposit 

 fine sand. This, therefore, appears to indicate a change in the course 



• Proceed. Roy Soc. 1862, p. 41. 



t Cyc. Brit. Article " Rivers," p. 274. 



