52 G. A. J. Cole Sf 0. H. Little— 



mostly yielded 2-755, but ranged from 258 to 2-79. Possibly some 

 aragonite or dolomite. 



Uoplites lautus, Gault, Folkestone. Two separate specimens, 2-75 

 and 2'79. Probably some iron pyrites present. 



Samites intermedins, Gault, Folkestone. 2"71. Calcite. 



Jptgchtis, Kimmeridge Clay, Ely. 2-688. Calcite. 



Belemnites giganteus, guard, Lias. 2*677. Calcite. 



Psiloceras planorhis, Lower Lias, Watchet. Six pieces from oue 

 specimen gave respectively 2-88, 288, 2-93, 2-94, 2-97, and 2-97. 

 One of the pieces giving 2 88 was then powdered; the bulk of the 

 powder floated at a level indicating 2'80. The powder ranged, 

 however, from 2-65 to 2-98. Internal mineral differences here 

 probably interfere with a true result, but the indication of aragonite 

 is remarkable. 



This last result gave cause for thought. Can aragonite be retained 

 in the shells of ancient deposits, and under what conditions ? Rose 1 

 has recorded aragonite crystals on the walls of the fibres of Inoceramus 

 Cuvieri from the Planermergel of Strehlen ; but it does not appear 

 tbat he traced shells consisting of aragonite into deposits of any 

 antiquity. Tschermak 2 remarks tbat the proportion of aragonite 

 decreases in older shells, although this mineral is found in fossil shells. 

 No further observations, however, are recorded by him on this point. 

 Messrs. Cornish & Kendall 3 deal mainly with the solution of aragonite 

 shells, rather than with their replacement by calcite. It is not clear 

 that Sorby 4 meant to assert that certain Gastropod shells in the 

 B arton Clay remained in the aragonite state. He calls attention to 

 the preservation of their structure, but is inclined to suggest partial 

 silicitication to account for it. As already stated, his use of the phrase 

 « aragonite shells" seems by no means to imply shells now consisting 

 of aragonite. 



It seemed to us important, then, to see how far " aragonite shells " 

 can remain as aragonite in geological time. "We have always to 

 remember in such an investigation that, when we meet with a fossil 

 shell of high specific gravity, there is the possibility of partial 

 phosphatization. This difficulty was indicated by Sorby, but has 

 been removed in recent years by the ingenious test devised by Herr 

 Meigen 5 to distinguish calcite from aragonite. Powdered aragonite, 

 as is now well known, turns lilac after boiling in a solution of cobalt 

 nitrate; while calcite, unless extremely finely powdered, gives no 

 reaction. 



The standard solution used in the following experiments was 

 prepared by adding two volumes of water to one volume of a cold 

 saturated aqueous solution of cobalt nitrate. The time of boiling in 



1 ' ' ijber die heteromorphen Zustande der kohlensauren Kalkerde ' ' : Abhandl. 

 Akad. d. Wiss., Berlin, 1858, p. 80. 



2 Mineralogie, 1884, p. 418. 



3 "On the Dissolution of Aragonite Shells in the Coralline Crag," Geol. 

 Mag., 1883, p. 497; and "On the Mineralogical Constitution of Calcareous 

 Organisms ", ibid., 1888, p. 68. 



4 Op. cit., p. 77. 



5 Centralblatt fttr Min., etc., 1901, p. 577, and Ber. naturforsch. Gesell. 

 Freiburg i. Br., Bd. xv, p. 20, 1907. 



