50 G. A. J. Cole 8f 0. H. Little— 



mineral nature of many shells. Haidinger l observed that aragonite, 

 when heated, flew into fragments, which he believed to be calcite, and 

 Rose showed that these really consisted of calcite, with a specific 

 gravity of 2*709. Rose noticed how under certain conditions of 

 precipitation rhonibohedra of calcite were formed, while aragonite 

 was deposited under others, its globular forms changing in time into 

 calcite rhonibohedra. 2 It is easy to precipitate aragonite from lime 

 water with ammonium carbonate, and to note under the microscope, 

 as Meigen has done, 3 the gradual assumption of the calcite condition, 

 which involves, of course, a change of volume. The fact that the 

 pi'ecipitate consists of aragonite may now be determined by a prompt 

 use of Meigen's solution, to be referred to later. The precipitate must 

 be freed from ammonium carbonate by thorough washing, since this 

 salt gives in a solution of cobalt nitrate a fine violet deposit of cobalt 

 carbonate. 



The question with which the present paper is concerned is the rate 

 at which the change from aragonite to calcite takes place in fossil 

 shells during geological time. The shell is not free to destroy its 

 intimate structure and to group its molecules in new arrangements 

 unless it can break up entirely into a number of granules of calcite, 

 or unless the excess of calcium carbonate, present in the aragonite 

 state, but not required in the calcite state, can be accommodated some- 

 where, or removed in solution as the change goes on. 



Aragonite with a specific gravity of 2-y4 expands 8'088 per cent, 

 in turning into calcite with a specific gravity of 2 - 72. Such an 

 expansion is comparable in amount with that which accompanies the 

 formation of ice from water ; and it is quite possible that ordinary 

 pressures in sedimentary rocks are sufficient to restrain it. In any 

 case, where the change of mineral condition takes place in a fossil 

 shell without alteration of its bulk and form, it seems likely that 

 solution has been at work. It may be urged that the organic matter 

 occupies considerable space in the living shell, and that the hollows 

 left by its decay are sufficient to accommodate the molecules of 

 calcium carbonate when re-arranged in the calcite state. But this 

 would not apply to the numerous cases of the conversion of oolitic 

 grains from aragonite to calcite, and here removal of material by 

 solution seems the only explanation. 



It should be remarked that Rose 4 found that microscopic crystals of 

 aragonite, which he obtained by evaporation of a solution of calcium 

 carbonate in water containing carbon dioxide, retained their form even 

 after heating, although their specific gravity of 2*700 showed that 

 they had passed into the calcite state. Rose described them as true 

 pseudomorphs, in which an expansion and re-arrangement of the 

 particles had taken place. But such an expansion of every oolitic 

 grain and shell-fragment in an oolitic limestone seems improbable. 



1 " Ueber die Veranderungen, welehe gewisse Mineralien, etc." : Poggendorff's 

 Annalen, Bd. xi, p. 177, 1827; also Trans. Boy. Soc. Edin., 1827. 



2 Op. cit., 1856, p. 1. 



3 "Beitrage zur Kenntnis des kohlensauren Kalkes " : Ber. naturforsch. Gesell. 

 Freiburg i. Br., Bd. xv, p. 1, 1907. 



4 Poggendorff's Annalen, Bd. xlii, p. 363, 1837. 



