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 renovating or conservative. The animal and vegetable remains of the ancient 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 freshwater formation in this country, as well as in France, is a circumstance 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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