FORCHHAMMER ON THE FORMATION OF DOLOMITE. 49 



The limestone from Faxoe contains — 



Carbonate of lime 98*246 



Carbonate of magnesia .... 0*924 



woi) yinnomP^^f^ «^ ^^^^ ^'it^ 



-Bvri. 'uu[-7 - '^^ ^^-^^ manganese O'2/o 



" ' ' Insoluble matter 0*399 



100-000 



This contains therefore barely 1 per cent, of carbonate of magnesia. 



The shelly Anthracite from the Transition Formation at Bornholm 

 contains — 



Carbonate of lime 91*62 



Carbonate of magnesia 1*02 



Insoluble in acid, and precipitated by ammonia . . .5*47 



Organic matter, water, and loss 1'89 



100*00 



Hereto belongs also the Phryganea-limestone from the base of Ger- 

 govia, near Clermont, in Central France, which consists of small fresh- 

 water shells, composing the tubular cases of larval insects, similar to 

 the cases constructed by the larvae of Phrygaiiea granclis and other 

 species of this genus at the present day. It contains — 



Carbonate of lime 91*52 



Carbonate of magnesia 1*01 



Insoluble matter 2*24 



Iron and manganese 0*58 



Organic matter, water, and loss 4*65 



100*00 



Other limestones, composed of shells and corals, present similar 

 proportions ; the small quantity of carbonate of magnesia present 

 being derived from the organic substances that have also supplied 

 the carbonate of lime. 



I have, for another purpose, analysed a large quantity of corals and 

 shells of marine animals with the following results : — 



Astraea cellulosa contains 0-542 per cent, of carbonate of magnesia. 



Myriazoon truncatum ... ^0'445 



Heteropora abrotanoifles ... 352 



Eschara foliacea ... 0-146 



Frondipora reticulata ... 0*596 



Corallium nobile ... 2-132 



Isis hippuris . . . 6*362 



If we omit the family to which Corallium and Isis belong, the pro- 

 portion of magnesia amounts to about \ per cent, of the mass of the 

 corals ; and when they pass over to the fossil state, it becomes pro- 

 portionally increased, as the organic constituents disappear. 



