50 



GEOLOGICAL. MEMOIRS. 



Of bivalves I have determined : — 



TerelDratula psittacea contains 0-457 per cent, of carbonate of magnesia. 



Modiolus papuana ' ... 0*705 



Pinna nigra, from the Red Sea, ... I'OOO 



Of univalves : — 



Tritonium antiquum 

 Cerithium telescopium 



Of cephalopods : — 



Nautilus pompilius 

 Sepia {ossa Sepioe) 



Of annelids : — 



Serpula, sp. from the"! 



Mediterranean, J 



Serpula triquetra, froml 



the North Sea, J 



Serpula filograna 



contains 0-489 per cent, of carbonate of magnesia. 

 0-189 



contains 0-118 per cent, of carbonate of magnesia. 

 0-401 



contains 7*644 per cent, of carbonate of magnesia. 



4*455 

 1-349 



If, therefore, we except the Corallium and Isis family from amongst 

 the corals, and Ser])ula from amongst the other lower, hme-secreting, 

 marine animals, the quantity of carbonate of magnesia, found in these 

 shells, remains in general under 1 per cent. We can, therefore, easily 

 explain why limestones composed of such animals usually contain a 

 similar proportion of magnesia. 



If, then, we fix the limit of the proportion of magnesia that a 

 hmestone may contain without losing its character of a '' common 

 limestone^' at 2 per cent., we shall have that Umit which is deter- 

 mined by the circumstances under which limestones are naturally 

 composed, namely by the remains of lime-secreting aquatic animals. 



If a Hmestone contains more than 2 per cent, of carbonate of 

 magnesia, I term it ^' dolomitic limestone."' It follows from the above 

 observations that SerpulcE can necessarily form dolomitic limestones 

 that are considerably rich in magnesia ; the same holds good with 

 Corallium and Isis, and probably some other families of animals. 

 The exact boundary between the ^'common" and the "dolomitic 

 hmestone " is somewhat loose and arbitrary, as the presence of dif- 

 ferent species and varieties in greater or less proportions must be 

 usually accidental ; and the same may be said of the boundary-line 

 between the ''dolomitic hmestone" and the ''dolomite." 



That rock, however, I call "dolomite," in which the proportion 

 of carbonate of magnesia is more than 13 per cent. ; and shall 

 hereafter give my reasons for this detemiination. Thus, then, "do- 

 lomitic lim;estone" will have between 2 and 13 per cent., and " dolo- 

 mite" above 13 per cent, of carbonate of magnesia. 



There are some naturalists who would consider the character of do- 

 lomite to be dependent on its granular ciystalline structure, as well as 

 on the proportion of magnesia present ; but Werner and Hoffman have 

 alread}' a compact dolomite, and others follow this example. Dolo- 

 mite in this respect is subject to the same conditions as limestones ; 

 it has its compact and its granular crystalline varieties. Other mi- 



