Calcium Carbonate in Fossil Shells. 51 



In determining the specific gravity of modern shells, the organic 

 matter presents a difficulty, as was, of course, recognized by Sorby. 1 

 Sorby estimated the specific gravity of this matter as 1*5, and obtained 

 its proportion by weight by carefully burning it off, and restoring, by 

 treatment of the residue with a solution of ammonium carbonate, any 

 carbon dioxide that might have been lost. The specific gravity of the 

 shell in its original condition, the masses of its two constituents, and 

 the density (1*5) of one of them are known; the density of the other, 

 the calcium carbonate, can then be calculated. 



Professor "W. J. Sollas 2 shows that 12-5 per cent, of organic matter 

 will reduce the specific gravity of a shell from 2 - 9 to 2 - 725, while 

 14'37 per cent, will bring it to 2*7, a value commonly afforded. 



When Professor E. H. L. Schwarz 3 argued in 1894 that the shell 

 of the modern Nautilus consisted of calcite, since its specific gravity 

 was 2 - 68, he may have been misled by the presence of organic matter. 

 As soon as his paper was published, one of us obtained a similar result for 

 Nautilus (2 1 '70), while the shell of Spirula was found to have a density 

 lower than 2*60. The organic jelly obtained on dissolving the latter 

 shell was, however, very conspicuous. It seems possible that the 

 value obtainable may vary in modern shells according to the conditions 

 to which they have been subjected, and that long exposure to sunlight 

 and the attack of organisms on a tropical beach may reduce them to 

 the state of fossil shells. It is clear, however, that organic matter 

 may remain present for a long time, since bleached shells of Mya 

 gathered from a raised beach in Green Bay, Spitsbergen, yield 

 a considerable amount of jelly on solution. 



Professor Schwarz proceeded to argue that the shells of ammonites, 

 which are often missing, and which are commonly formed of calcite 

 when preserved, originally consisted of calcite, like their aptychi. 

 Sorby, however, had already stated the specific gravity of the shell of 

 Nautilus as 2*95, 4 and had suggested the presence of a little calcium 

 phosphate in addition to aragonite. The internal shell of Sepia 

 yielded him 2*91. Hence the ammonites also have been generally 

 held to have had shells of aragonite, which have passed gradually 

 into calcite. 



Using Sollas's diffusion column and methylene iodide, one of us 

 made the following determinations on fossil shells in 1894, calcite and 

 aragonite being the floating indices in the liquid : — 



Nautilus, Grault, Folkestone. 2-73. Calcite. 



Ccelonautilus carinatus, Carboniferous Limestone, County Limerick. 

 2'68. Calcite. 



Cosmoceras Jason, Oxford Clay, Chippenham. 2 , 67o. Calcite. 



Harpoceras Brighti, Oxford Clay, Christian Malford. Would not 

 sink until powdered, probably through air-cavities. The powder 



1 Quart. Journ. Geol. Soc, vol. xxxv, Proceedings, p. 59. See also Kose, 

 Abhandl. Akad. Wiss., Berlin, 1858, p. 68. 



2 ' ' On the Physical Characters of Calcareous and Siliceous Sponge-spicules 

 and other Structures " : Sci. Proc. E. Dublin Soc, vol. iv, p. 390, 1885. 



3 " The Aptychus " : Geol. Mag., 1894, p. 457. 



4 Op. cit., p. 60. Cf. Cornish & Kendall, "On the Mineralogical Constitution 

 of Calcareous Organisms " : Geol. Mag., 1888, p. 71. 



