■234 Mr. Webster on the Strata lying over the Chalk. 



buildings constructed with flints that have resisted the agency of 

 the atmosphere for many centuries without undergoing the smallest 

 visible alteration, or having become whitened in the least degree. It 

 is however well known to be liable to decomposition under certain 

 circumstances ; and it must be allowed that the combined action of 

 moisture and various decomposing causes whilst a mineral remains 

 buried in the soil, may produce effects which we can scarcely estimate. 



Upon the whole, however, it appears to be extrem.ely improbable, 

 that any species of imaginable action could have converted a 

 fragment of chalk flint into a substance so very different as one of 

 the rounded concentric pebbles of the London gravel. 



To assist us in endeavouring to obtain a just-idea with respect 

 to the origin of the different accumulations which are found in our 

 gravel, it may be useful to consider the various changes which 

 have taken place in our upper strata. Of these, although ignorant 

 of their causes or their extent, we yet perceive the traces written 

 in characters sufficiently legible. 



Although the chalk has been originally formed at the bottom of 

 the ocean, yet from some change which took place either in the 

 level of the sea, or in the state of the strata, part of it probably ai: 

 an early period has been above, and part below, the surface of the 

 water, as at present ; and this before the deposition of those strata 

 which we now see immediately superimposed upon it. 



From that date, and by the same cause as we see still producing 

 this effect, did probably the formation of rounded flint pebbles 

 begin. The chalk itself, being easily acted upon by the waves, 

 became disintegrated, while the siliceous nodules were better able 

 to resist this abrasion, though yet liable to be broken and rounded 

 by friction against each other. 



This effect takes place chiefly upon the margin of the sea. In 



