54 Cole Sf Little — Calcium Carbonate in Fossil Shells. 



Upper Chalk is calcite, with a specific gravity of 2-70. "We have 

 verified this with Meigen's solution, and regret that it is difficult 

 to find other suitable fossils for comparison. Spondylus, for instance, 

 possesses an outer calcite layer, while Ostrea and Pecten, both common 

 in the Chalk, form calcite shells. The Cephalopods, other than 

 Pelemnites, from the Chalk are usually in the form of casts, and there 

 is a general dearth of material that might have been aragonite at its 

 time of deposition. From the Upper Greensand of Devizes, however, 

 we have a specimen of Area carinata, which has the decayed and 

 crumbly look of an old aragonite shell ; and this proves to be actually 

 aragonite. 



When we come to the Albian horizons, we possess numerous fossils 

 from the Gault Clay which have remained in the aragonite condition. 

 The following are all aragonite : Folkestone, Cyclocyathus Fittoni, 

 Alaria carinata, Soplites tuberculatus, Soplites splendens (two 

 specimens) ; Mid-Surrey, JVucula pectinata (showing a characteristic 

 powdery appearance). On the other hand, the following from the 

 Gault of Folkestone have passed into calcite : Solarium ornatum, 

 Nautilus, Soplites lautus, Samites elegans. A Solarium again, from 

 an unrecorded Gault locality, has remained as aragonite. 



It is clear that local conditions have allowed a majority of Gault 

 shells to remain unaltered, while others have undergone change in 

 geological time. We may expect to find this change emphasized in 

 Jurassic strata. 



Jurassic. Coral Rag : West of England, Thecosmilia annularis, 

 Thamnastrcea arachnoides ; both calcite. Kelloways Rock : Christian 

 Malford, Cosmoceras Flizabethte, calcite. Bath Oolite : Minchinhampton, 

 Area hirsoniensis, Area Prattii, Area minuta ; all calcite. Inferior 

 Oolite : Parhinsonia Parkinsoni, calcite. Middle Lias : Cheltenham, 

 Kfaerodon Buckmani ; Lyme Regis, AEgoceras Senleyi ; both calcite. 

 Mickleton, AEgoceras capricornus, aragonite. Lower Lias : Robin 

 Hood's Bay, Psiloceras planorbis, calcite ; Watchet, Psiloceras 

 planorbis, aragonite. 



The interest here is concentrated on two ammonites. The AEgoceras 

 from Mickleton is very well preserved in a slightly argillaceous lime- 

 stone, while the Psiloceras from Robin Hood's Bay is also in a clayey 

 limestone. That from Watchet is in a shale ; it was this specimen 

 that raised the question of the retention of aragonite in ancient strata, 

 when its specific gravity was found, in 1894, to be as high as 29. 

 We have examined Jurassic Areas specially as a type, and it is clear 

 that there is a very general tendency for " aragonite shells" of this 

 period, when they survive at all, to be preserved as calcite. 



Trias. Red marine strata of Han Bulog, Bosnia, Ptychites striato- 

 plicatus, calcite. 



Carboniferous. Carboniferous Limestone, Ccelo?iautilus carinatus, 

 calcite. Shale, Cuilcagh, Orthoeeras, calcite. 



Finally, we have verified Sorby's observations on Oolitic limestones, 

 with the foreseen result that, while the oolitic grains from the modern 

 Great Salt Lake of Utah consist of aragonite, those of Jurassic age 

 from Portland, Leckhampton, and Dalberg in Hanover have passed 

 into calcite. 



