DEVONIAN SYSTEM : PLATEAU LIMESTONE. 187 



From analyses made by the late Mr. T. R. Blyth, Assistant Curator, 

 in the Geological Survey Laboratory, of a 



Chemical analyses. . . . . . . . . . 



series of typical specimens of the limestone, 

 it appears that they vary from an almost pure calcite, containing 

 oyer 99 per cent, of that mineral, through dolomitic limestones, 

 with from 19 to 33 per cent, of magnesium carbonate, to rocks 

 containing the carbonates of lime and magnesia in the proportions 

 of 55 to 44 per cent, respectively, or practically speaking, true 

 dolomites. One point is to be noticed, however, namely, that the 

 pure limestones belong to the Permo-Carboniferous horizon o f the 

 Plateau Limestones, and that all those from the massive, non-fos- 

 siliferous bulk of the formation yield an appreciable amount of 

 carbonate of magnesia ; also that those with the lower proportions 

 of this substance contain a relatively large quantity of insoluble 

 residue {see Table 7). 



