406 C. L. lieese — Influence of Swamp Waters, etc. 



acids as absorption compounds, on coming in contact with the 

 carbonate of the marl, which forms the bottom of the swamps, 

 is deposited either as the mon-acid or normal salt (if the 

 former it changes into the latter in the course of time) and 

 carbon dioxide is liberated. This carbon monoxide in its 

 turn dissolves some of the carbonate, thus bringing about a 

 concentration and consequent tendency toward .nodulation as 

 will be seen from the reaction ; one molecule of the phosphate 

 would remain and four molecules of the carbonate would be 

 removed. 



2 CaC0 3 + H 4 Ca(P0 4 ) o = Ca,(P0 4 )„ + 2H o + 2C0 2 

 2 CaCO a + 2CO a + 2H 9 = 2H 2 Ca(C0 3 ) 2 ' 



The humus substances also give off carbon dioxide on 

 decomposition and consequently we have a further cause for 

 concentration from this source. 



There can be no doubt, from the similarity in structure, that 

 the phosphatic nodules are phosphatized marls. And from 

 their occurrence in beds in a country that has in recent geo- 

 logical history been covered from time to time with swamps, 

 some of which at present underlie a bed of argillaceous sand 

 covered by salt marsh, taken together with the above observa- 

 tions and experiments, it certainly seems probable that swamp 

 waters have brought about this change through the agency of 

 carbonic acid and the humus substances contained in them. 



The question as to the source of these large deposits of phos- 

 phates is of course a very difficult one to answer. There are 

 many fossil remains of animals found associated with the rock 

 but their numbers do not seem to be sufficient to account for 

 the large quantity of the*phosphate found. Again these fos- 

 sils (mainly sharks' teeth and bones) seem to be of a later date 

 than the nodules, as was suggested by Mr. Penrose, owing to 

 the fact that they are not embedded in the nodules but are 

 simply found loosely mixed in with them. Some of the phos- 

 phate is that which was originally in the marls, and I might 

 suggest as other sources, that brought down in solution by drain- 

 age, and a small quantity contributed by the potassium phos- 

 phate from the decomposition of vegetable remains, particu- 

 larly such accumulation in swamps. 



"The Citadel," Charleston, S. C, Jan. 20th, 1892. 



