THE 



GEOLOGICAL MAGAZINE. 



NEW SERIES. DECADE V. VOL. VIII. 



No. II.— FEBRUARY, 1911. 



ORIG-HST-AJL. ARTICLES. 



I. — The Mineral Condition of the Calcium Carbonate in Fossil 



Shells. 



By Grenvelle A. J. Cole, F.G.S., and Otway H. Little, A.B.C.Sc.L, B.A., 



Boyal College of Science for Ireland. 



A RECENT paper by Mr. A. R. Horwood on " Aragonite in the 

 Middle Lias " * raises again the question of the condition of the 

 calcium carbonate in fossil shells. Mr. Horwood speaks (p. 176) of 

 Jurassic shells as being " preserved in aragonite ", and quotes Dr. Sorby 

 as being of the same opinion. But is Mr. Horwood sure that his shells 

 are still in the aragonite condition ? Sorby, 2 when he writes of 

 "aragonite shells", means as a rule shells that were formed of 

 aragonite at the time of the death of the originating animal ; but he 

 describes how, in many of our fossiliferous rocks, such shells are 

 represented either by hollow spaces or by pseudomorphs in granular 

 calcite. 8 He points out that oolitic grains, originally consisting of 

 aragonite, have similarly been changed into calcite, the more stable 

 form of calcium carbonate. 4 Some oolitic grains, however, were 

 thought by him to have consisted o£ calcite from the outset. Doelter, 5 

 after summing up Linck's researches on calcite and aragonite, states 

 that calcareous oolites now consist of calcite ; and we have no doubt 

 that the "recrystallized aragonite concretions", quoted by Mr. Horwood 

 from Sorby, were held by Sorby to be at the time of examination in 

 the calcite state. 



Sorby and other workers before 1901 have been compelled to rely 

 upon the test of specific gravity and of relative hardness in dis- 

 tinguishing between calcite and aragonite in shells. A number of 

 interesting observations were brought together in a classic paper by 

 Grustav Rose, 6 who employed the microscope in determining the 



1 Geol. Mag., 1910, p. 173. 



2 Quart. Journ. Geol. Soc, vol. xxxv, Proceedings, pp. 65, 83, 93, etc., 1879. 



3 Ibid., p. 68. 



4 On trie relative stability of calcite, see H. W. Foote, "Uber die physikalisch- 

 claemischen Beziehungen zwischen Aragonit und Calcit " : Zeitschr. fur 

 plrysikalische Chemie, Bd. xxxiii, p. 753, 1900. 



5 Pedogenesis, p. 225. 



6 " Uber die heteromorphen Zustande der kohlensauren Kalkerde " : Abhandl. 

 Akad. d. Wiss., Berlin, 1856, p. 1, and 1858, p. 63. Bose's first researches on 

 the mineral forms of calcium carbonate were published in Poggendorff's 

 Annalen, Bd. xlii, p. 353, 1837. A French translation, containing a correction 

 of a slip on p. 361 of the German paper, appeared in the Annates des Mines, 

 1837, p. 611. 



decade v.— vol. vm. — no. h. 4 



