Mr. Webster oh the Strata lying over the Chalk. 251 



It would seem to have been a circumstance accompanying the last 

 great revolution which the earth has undergone, that siliceous earth 

 has been held less abundantly in solution since that period. That 

 event appears to have been accompanied by a process of destruction 

 merely ; but former changes were alternately destructive and reno- 

 vating or conservative. The animal and vegetable remains of the 

 ancielit world are frequently impregnated with siliceous matter. 

 But I believe no well authenticated instances can be adduced of such 

 a process going on in our times. Petrifaction, indeed, in the proper 

 sense of the word, seems now to have entirely ceased. 



In the strata over the chalk in France silicified organic bodies are 

 abundant : in our upper strata they are rare, if we except those 

 found in the gravel whose original situation is yet questionable. 

 Indeed the only instance with which I am acquainted in this country 

 in the strata over the chalk, are the siliceous fossils of Feversham 

 already mentioned : some of these are entirely calcedonic. 



The existence of the marine strata placed above the lower fresh- 

 water formation in this country, as well as in France, is a circum- 

 stance much more difficult to explain, and would seem to require 

 either a rising of the sea or a sinking of the land in this part of the 

 globe. 



Alterations In the shape of the coasts, and the accumulation of 

 sand and pebbles in various parts of the sea, affect the tides so 

 considerably, as to occasion them to rise to very different heights at 

 the same places at different periods ; yet no change of this kind can 

 be Imagined sufficiently great to account for an effect so considerable 

 as has been produced. 



Instances of marine strata placed over those formed in fresh water 

 have been observed not only here and in the basin of Paris, but in 

 other places. One is mentioned by Professor Herman, of Strasburgh. 



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