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HUTTONIAN THEORY. 



191 



confidered in what Hate they are found. In 



r 



general, they have a fparry ftrudure, very dif- 

 ferent from that of the original fhell or coral, 

 of which, however, they retain the figure with 

 wonderful exadnefs, though probably fome- 

 times altered in lize. Though fparry, they are 

 often foliated, and preferve their animal, in con- 

 jundion with their mineral, texture. Now, this 

 cryftallization is a mark of fome operation, 

 quite different from any that can be afcribed 

 to the water in which thefe bodies had their 

 origin, and by which they were brought into 



their place. 



They 



were impervious to wa- 



ter ; and it cannot be faid that their fparry 

 ftrudure has been derived from the percolation 

 of that fluid, carrying new calcareous matter 

 into their pores. We can account for the 

 change produced in them, I think, only by 

 fuppofing them to have been foftened by heat, 



fo as to permit their parts to arrange them- 

 felves anew, and to aliume the charaderiflic 

 organization of mineral fubllances. 



All Ihells have not the change effeded on 

 them that is here referred to ; thofe in chalk, 

 for inftance, retain very much their original 



This 



IS 



what we might 



form in all refpecls. 



exped from the very different degree of inten- 



sity, 



with which the mineralizing caufe has 

 aded on chalk, and on limellone or marble. In 



^ 



general, it is in the hardefl and moil confolida- 



ted 



