THE CHERTY LHIESTONE. 

 Analyses of limestones. 



131 



SiOj 



TiOi 



Al.O, 



F&2O3 



FeO 



MnO 



CaO 



MgO 



HjO 



CO.2 



pjOs .: 



SO3 



CI 



FeS, 



Organic matter. 



No. 1. 



3-07 



Trace. 



99 IS 



No. 2. 



0-63 



Trace. 



99-65 



Dolomite and calcite are not readily distinguished in the thin section. 

 So far, however, as appearances go, the carbonate entering into the compo- 

 sition of these rocks seems to be always the same, and, judging from the 

 content of magnesium carbonate, it is rather to be counted as dolomite than 

 as a mixture of dolomite and calcite or as a magnesian calcite, thougli the 

 assertion that calcite is always absent is not ventured. Under the micro- 

 scope the dolomite individuals, which constitute the larger part of each 

 section, present the usual appearance met with in sections of crystalline 

 limestone, being without crystal outlines, but fitting together closelv along 

 irregular curved lines. The interlocking of the grains is however, usually 

 somewhat greater than in the case of statuary marble. 



In addition to the carbonate, most of the sections show more or less 

 tremolite, which varies greatly in quantity. Occasionally only a few 

 minute flakes are discoverable, while in other cases broad single blades of 

 radiating clusters make up a large portion of the thin section. The tremo- 

 lite blades often penetrate the carbonate in every direction, and in the case 

 of the larger blades the dolomite appears to be included within the tremo- 

 lite. In yet other cases the tremolite in aggregated blades appears to 

 make up the whole of certain narrow belts in the thin section. The tremo- 



