238 Mr. Webster on the Strata lying over the Chalk. 



If therefore we admit that the pebbles of our gravel are derived 

 from the destruction of former strata, that many of them differ from 

 any other siliceous bodies with which we are acquainted, and that 

 these bear perhaps a stronger resemblance to the flints of the Paris 

 beds than to those of the chalk (judging from some specimens 

 of French flints in the Museum of the Geological Society), and if 

 we have reason to believe that some of those beds formerly existed 

 in this country, will it be considered as a conjecture rash and un- 

 warranted, should I imagine that the substances in question have 

 derived their origin from siliceous nodules, originally formed in 

 strata which existed over our blue clay, but which have been dis- 

 integrated, and carried off, in one of those revolutions to which 

 this part of the earth has been subjected ?* 



The formation of the nodules of flint in chalk has frequently, 

 and will probably much longer continue to excite the speculations 

 of philosophers. But whatever was the mode in which they origi- 

 nated, we may fairly imagine that these siliceous masses in question 

 now found in our gravel, (but which I have supposed to have 

 belonged originally to regular strata) were formed in a similar 

 manner. 



The coats of the flints in chalk have been found by chemical 

 analysis to consist of chalk mixed with the flint j and it appears that 

 all nodules formed in a matrix have crusts or coats, composed of a 

 combination between the matter of the nodule and the enveloping 

 substance. 



The coats of these pebbles may be supposed therefore to exhibit 

 a similar phenomenon ; and in them we may perhaps see a part of 

 the stratum which they were at first imbedded in. 



* The Fgyptian pebbles are said io occur in a similar situation. 



