87'Z On the Composition of the Cambrian and Silurian Limestones. 



more easily decomposable compounds, and admirably fitted for 

 yielding up lime to the action of water and carbonic acid. 



Under the impression that the occurrence of magnesia in 

 limestones first commenced during the Devonian period, Kjerulf 

 has recently made use of this view as a means for determining 

 the position of stata, which, not being fossiliferous, do not afford 

 palfeontological evidence of their geological position. The 

 desirability of such a means of classification is obvious, but its 

 correctness must be questioned. 



The analyses here brought forward certainly show but very 

 little magnesia ; but I have found that two limestones imbedded 

 in the metamorphic schists west of Krageroe, in Norway, are true 

 dolomites, and from their position must be Cambrian. Sheerer 

 has also noticed the occurrence of much carbonate of magnesia 

 in the metamorphic rocks near Snarum in Norway ; and dolo- 

 mites occur in the Silurian rocks of Livonia, Esthland and 

 Kurland ; and I have reason for supposing that some of the 

 rocks which, from the occurrence of dolomites in them, are 

 classed by Kjerulf as Devonian, may possibly prove to be 

 Cambrian. 



We must also remember that the presence of magnesia is not 

 necessarily connected with the magnesian limestone formation ; 

 as we know that mountain limestone in some parts of Leicester- 

 shire, Derbyshire, Yorkshire and Northumberland is highly 

 magnesian ; on the other hand also, many beds of the magnesian 

 limestone series do not contain more than a trace of magnesia, 

 although in the midst of dolomites. 



The two following analyses, by Liebe, of German limestones 



prove this : — 



Thieschiitz. Schwara. 



Carbonate of lime . . 86-09 93'45 



Carbonate of magnesia . 0'73 0-69 



Carbonate of iron 1'07 



Insoluble 11-96 3-21 



Water 0-57 



Loss 0-65 1-58 



100-00 100-00 



Both of these were alternating with true dolomites. 



On the whole, the evidence brought forward with reference to 

 the occurrence of magnesia in limestones seems to favour the 

 view that its presence is due to subsequent alteration, probably 

 by infiltration, as may be proved in many cases. If so, we may 

 expect magnesian limestones to be of all ages. 



How far organic life is capable of assimilating magnesia in 

 place of lime is not sufficiently investigated, but the presumption 



