PROCESS OF PETRIFACTION. 63 



The same is true of shells imbedded in sand. The carbon- 

 ate of lime will gradually dissolve, and the silica held in 

 solution, will slowly, atom by atom, take its place, and we 

 have a petrified shell that has completely lost its original 

 carbonate of lime, and become perfectly silicified, though its 

 form and outline remain the same as it grew. And so where 

 the nodules were buried in sand, as they were in some portion 

 of the Santee marl-beds, as we see them about Aiken, they 

 were changed into buhrstone, or beds of silicious shells, 

 having lost every particle of their original composition as a 

 carbonate of lime, and become so solid and hard as to make 

 the best of millstone, for which this rock is often used. The 

 buhrstones imported from France, and used as millstones, 

 are precisely similar. But these nodules, instead of being 

 imbedded in and covered with sand, in which case they would 

 have changed into a hard silicious rock, were buried with a 

 great mass, consisting of the fecal and other phosphatic 

 remains of animals. As their carbonate of lime was, atom 

 by atom, dissolved out, its place was taken by the phosphoric 

 acid filtering down from above, and this process, going on 

 slowly age after age, the whole mass was changed into a 

 phosphate instead of a carbonate of lime. And thus we find, 

 in this wonderful laboratory of nature, a vast and almost 

 incredible store of material, which, by a simple and easy 

 process of manipulation, can be readily converted into the 

 food of plants, so useful to man and the domestic animals 

 subservient to his purposes in the attainment of the highest 

 forms of civilization and culture. 



If this theory of the history of the phosphate rocks is 

 correct, it still follows that they were of animal origin, though 

 not in the form of bones. The marl itself may be said to be 

 of animal origin, having been deposited by marine shell-fish, 

 which, in a myriad of forms, swarmed in the warm and 

 genial waters poured along over this shallow sea by the Gulf 

 Stream. In fact, the city of Charleston itself "is built upon 

 a bed of animalcules several hundred feet in thickness, every 

 cubic inch of which is filled with myriads of perfectly pre- 

 served microscopic shells." To the naked eye, the strata of 

 marl underlying the city may appear like some variety of 

 common earth, but the microscope reveals the fact that the 



