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I DO not by this pretend that the fecondary ftrata had not ac- 

 quired a certain degree of folidity in a few minutes, even after 

 their depofition, and confequently long before their emerfion ; on 

 the contrary this muft foon have been acquired in the fame manner 

 as we find calcareous depofits to harden at the bottom of falt- 

 pans and tea-kettles, while full of liquor, and tartar in hogfheads 

 of wine, and pouzzolana mortar, &c. but this hardnefs is mo- 

 derate in comparifon of that which they acquired by deficcation 

 and continued infiltration, as we daily experience in moift lime- 

 ftone quarries, where though the ftone is originally hard, yet it 

 becomes much harder when dried. The induration effedled by 

 infiltration is ftill more confiderable, as by it the minuter par- 

 ticles of bodies are conveyed into the minuteft interftices. Hence 

 we fee that traps and bafaltic pillars are always harder ^t the 

 bottom than at the top, Cronft. § 267, and the upper ftrata of 

 limeftone, particularly of that fpecies called freeftone, are always 

 fofter near the furface of the quarry than at a greater depth; that 

 fuch infiltration is not an imaginary procefs, let it be explained 

 how it may, and confequently that the hardnefs of the lower ft;rata 

 does not proceed merely from the prefigure of the upper, appears 

 by an elegant obfervalion of Mr. Werner's, that where various 

 ftrata of a different nature occur, the petrifadions that are found 

 in the inferior, are frequently filled with the matter of the fupe- 

 rior inftead of that of the ftratum which contains them, Wedem. 

 Verwandl. 118. The petrified fliells found in clay or argillites 

 are commonly compreffed and flatted, as Mr. Bergman remarks, as 



argil 



