ARCHAEAN. 



39 



the process, according to M. Lacroix, would be the reverse of that 

 which is supposed by Professor Judd to have taken place in the form - 

 ation of the Kuby Mines limestone. 



An important contribution to the discussion of the origin of 

 such crystalline limestones has been furnished 



Crystalline limestones , _ T _ J, , T ., 



in the Dharwar system, by Dr. L. L. termor. 1 In this paper the 

 genesis of the limestones is attributed to 

 causes similar to those put forward by Professor Judd, that is, they 

 are considered to have been formed by the alteration of pre-existing 

 rocks containing lime and magnesia silicates, the process necessi- 

 tating the introduction from without of carbon dioxide dissolved 

 in heated waters. Dr. Ferm.or was led to this conclusion by the 

 similarity between the mineral constituents of the calciphyres and 

 of the quartz-pyroxene gneisses of the district, and though he does 

 not go so far as to say that the limestones have been derived 

 directly from the gneisses by chemical alteration, he thinks that 

 they may have originally possessed practically the same composi- 

 tion. Since this paper was written, however, Dr. Fermor has 

 considerably modified his opinion, and in order to account for the 

 presence of carbon dioxide in sufficient quantities to effect the 

 change into crystalline limestone, he supposes that in the first in- 

 stance the Dharwar rocks of Chhindwara consisted of sediments, 

 partly calcareous ; that these were converted, under conditions of 

 high temperature and pressure, into calcareous gneisses, with the 

 expulsion of large quantities of carbon dioxide ; that the gas so 

 expelled was kept in solution at a high temperature and pressure 

 in the waters impregnating the rocks ; and finally, that as soon as 

 these conditions abated, the waters, charged with alkalies in addi- 

 tion to the carbon dioxide, began to attack the recently formed 

 gneisses, and to re-convert them into limestones. Dr. Fermor' s 

 ultimate conclusion is, therefore, that whatever may have been the 

 changes through which the crystalline limestones of Chhindwara 

 have passed, they may have been in the first instance sedimentaiy 

 deposits ; whether these were formed by chemical deposition, or 

 whether the carbonate of lime was accumulated by living organisms, 

 is another question, but it is one that has no particular bearing 

 on the present argument. 



1 Notes on the Potrology, etc., of Glihindwara District, Central Provinces ; Records. 

 Oed. Surv. Ird., Vol. XXXIII, Pt. 3, p. 150. 



