78 J. G. Gooclvhild — The Paste of Limestones. 



and Woodhead have not dealt : —It is a well-known fact that solutions 

 of sulphate of lime in the presence of decomposing organic matter 

 tend first to be reduced to the sulphide, and ultimately to be thrown . 

 down as a precipitate of carbonate of lime. In a paper on " Some 

 Modes of Formation of Coal," read before the Royal Physical Society 

 of Edinburgh, on April 17th, 1889 (published in the "Colliery 

 Guardian," May, and, in a different form, in the Geol. Mag. July, 

 1889), reference was made to this factor in connexion with lime- 

 stones of inorganic origin. Such limestones would necessarily 

 consist almost entirely of an amorphous calcareous paste in which 

 animal organisms might or might not occur, although bituminous 

 matter due to the partial or the complete decomposition of vegetable 

 organisms might be present in variable quantity. In the field, lime- 

 stones of this nature are of common occurrence in connection with 

 beds of coal, as might be expected on the view of the origin of 

 both these rocks advocated in the papers referred to. From such 

 inorganic limestones a passage can readily be traced in one direction 

 through clay ironstone, and blackband ironstone, into coal ; while 

 in the other direction (presumably that farthest from the land) such 

 limestones gradually pass, by the addition of imbedded calcareous 

 organisms, into limestones of the normal type. This led me, by 

 inference drawn from another set of facts, to the conclusion that 

 chemical precipitates, due to the reaction of decomposing organic 

 matter upon sulphate of lime, have an essential influence upon the 

 formation of limestones, even when these are mainly of organic 

 origin. Wherever organisms are living, at that place also are other 

 beings that are passing into the inorganic condition. In the case of 

 those of marine habitat, it has been shown that these secrete carbonate 

 of lime out of the sulphate of lime of sea-water while they are 

 living; and the products of decomposition bring about a precipitate 

 carbonate of lime from the same source when they are dead. Such 

 action is not by any means necessarily limited to the tenants of the 

 sea-floor, for the decomposition of pelagic organisms is likely to 

 contribute more or less to the same result. Nor is it limited to 

 organisms with calcareous frameworks, but may result just as much 

 from the decay of, say, jelly-fishes, or sponges, as from their lime- 

 secreting allies. 



It is to this organico-chemical agency that I would refer the origin 

 of the greater part of the paste of limestone of marine origin. 



The shell-marl of lakes presents limestone paste under a some- 

 what different form. Even in this case the bulk of the rock is 

 derived partly from the precipitation, by decomposing organic 

 matter, of the carbonate of lime from the sulphate carried in solution 

 into the lakes ; partly from the precipitation of the carbonate of lime 

 directly from solution by the withdrawal of part of the solvent 

 carbonic acid by the agency of subaqueous vegetation ; partly also 

 by the actual decomposition of the shelly matter itself, under the 

 •corroding influences of the acidulated waters. Shells themselves, as 

 a rule, of course, form but a small part of shell-marl. 



If the views here advocated find acceptance amongst geologists, it 



