( -240 ) 



taming carbonic acid than calcite. At the same time it is evident 

 however thai the difference of solubility is too small to serve for a 

 practical remedy of determination. 



2. that this little difference is not sufficient to explain the different 

 behaviour of the fossil lime-shells, but thai we must be taken into 

 consideration still other causes: the long- duration of the dissolving 

 action, the continuous supply of new liquor, the absolute size of 

 the shells and the relative size of the outer surface. 



One finds in the common test-books the notice that LOO grams of 

 water containing- carbonic aci 1 dissolve 0,1 or 0,12 grams of calcite 

 which agrees sufficiently with the number mentioned above l ). 



When examining the mineralogical composition of some shells, the 

 question rose what remedy is the most suitable to distinguish calcite 

 from afagonite practically. The little difference in hardness is of no 

 use to this purpose, as a small quantity of silex may neutralize this 

 difference. A calcite shell for instance may equal locally the hardness 

 of aragonite by mixed silex and a determination depending only on 

 this property seems precarious. Nor is the specific weight to be used 

 as a certain quantity of silex, phosphatic lime, magnesia carbonate 

 and organic materials inlluences it. On the contrary the test with 

 dilute cobaltcarbonate solution of W. Metgen is very useful (Central- 

 blatt für Mineralogie, Geologie und Palaeontologie, Jahrgang 1901, 

 Seite 577). With the aid of this test I examined a number of* shells 

 of the following genera, availing myself in some dubious cases of 

 the optical properties. 



Of calcite consisted : 



Ostrea (recent, pliocene and senon, 



Pecten (recent, pliocene and senon) 



Pectunculus (miocene) 



Area (miocene) 



Nucula (miocene) 



Leda (miocene) 



Venus (miocene) 



('vtherea (miocene) 



Isocardia (miocene) 



Littorina (recent) 



Buccinum (recent) 



Aporrhais (miocene) 



Ancillaria (miocene) 



] ) Here it must still be added that all these experiments took place at the common 

 chamber temperature (15° or 2(T G.) and that the glasses were shaken once a 

 day in order to promote the action of the dissolving liquor. 



