Calcium Carbonate in Fossil Shells. 53 



each case was three minutes. The first materials tested, to establish 

 a standard method, were powdered calcite and aragonite. 



Chemically prepared tricalcic orthophosphate gives a fine violet 

 reaction, and this has led one of us into the error of stating that 

 apatite " gives a blue precipitate ", l Several varieties of apatite and 

 phosphatized fossils tested by us are, as a matter of fact, found to 

 give no reaction, and this clears the way as regards the occurrence of 

 calcium phosphate in shells the specific gravity of which resembles 

 that of aragonite. 



Since Meigen mentions only results with artificially prepared 

 strontium and barium carbonates, it may be useful to state that we 

 find that two specimens of strontianite, one from the Harz and one 

 from Westphalia, give a blue reaction, while witherite turns violet 

 like aragonite. Magnesites from Haute Vienne and Silesia and 

 siderite from Cornwall give no reaction. 



Seeing how colour-changes occur unexpectedly in certain reactions. 

 we tested pyrite, marcasite, and a number of pyritized fossils, but 

 without result. We may conclude, then, that the violet reaction on 

 which we rely in our observations is a true indication of aragonite. 

 The presence of witherite is too improbable to vitiate the argument. 



Among Recent specimens we tested Nautilus pompilius and Area 

 Noce ; both show the aragonite reaction. Tubipora retains its red 

 tint, but gives no indication of aragonite. 



Pliocene. Professor Kendall 2 showed that aragonite shells may 

 be removed altogether in solution from the permeable Coralline Crag. 

 Those that remain are not likely to have been converted into calcite. 

 The following were tested and consist of aragonite. Chillesford Beds : 

 My a arenaria. Red Crag : Card hem Parkinsoni. 



Miocene. Burdigalian : Leognan, Area Girondica and Turritella 

 terebralis are both aragonite. 



Oligocene. Aquitanian : Merignac, Area cardiiformis, aragonite. 



Eocene. The "aragonite shells" sometimes show friability and 

 indications of partial solution ; but they seem able to hold their own 

 in a remarkable degree against transformation into the calcite state. 

 The following were tested, and all seem to be in their original mineral 

 condition. The one calcite shell in the series is Ostrea submissa, from 

 the Ypresian of Cuise-la-Motte; but, as is well known, the oysters form 

 their shells of calcite. All the following are aragonite. Bartonian : 

 Haravilliers, France, Area biangula. Lutetian : Chaumont-en-Vexin, 

 Eupsammia trochiformis ; Bracklesham, Litharcea Webster i and Area 

 duplicata ; Chaumont-en-Vexin, Cardium porulosum and Turritella 

 carinifera. London Clay : London, Nautilus regalis. Lower London 

 Tertiaries (Thanetian) : Chislehurst, Cyrena cuneiformis. 



Cretaceous. Gustav Rose 3 found crystals of aragonite on the fibres 

 of Inoeeramus Cuvieri from the Planermergel of Strehlen. Messrs. 

 Cornish & Kendall 4 determined that Parasmilia centralis from the 



1 Aids in Practical Geology, 6th ed., p. 36. 



2 Geol. Mag., 1883, p. 497. 



3 Abhandl. Ahad. Wiss., Berlin, 1858, p. 80. 



4 Op. cit., Geol. Mag., 1888, p. 72. 



