9 6 



Cincinnati Society of Natural History. 



Si 2 , 73 69 



Al 2 3l . . . 12.89 



Fe 2 3 , 1.02 



Fe O, . . V . 2.58 



CaO, 3.74 



MgO, , . . v , .50 



Na 2 O . . . . 2.81 



K 2 O, 1.48 



H 2 O, . - « . 1.06 



Total * 99.74 



The proportionate mineral composition was calculated from 

 this analysis in the same way as that of the Rowlandville 

 granite, and the remarks made on that calculation apply like- 

 wise to this one. Since the epidote occurs only in small 

 amount, one per cent (ifc) of the calcium was arbitrarily 

 assigned to this mineral, and the remainder is thought to 

 belong to the anorthite molecule. From this calculation, 

 taken in connection with the specific gravity separation, and 

 a study of the relative areas occupied by these minerals in 

 the thin sections, the following percentages are thought to 

 fairly represent the rock : 



Calculated Percentages, From 

 Chemical Analysis. 



Quartz 40.0 



Orthoclase 9.0 



Albite molecule \ 23.8 



Anorthite " j 13.6 



Percentages Obtained by Specific 

 Gravity Separation. 



Sp. gr. 

 2.65 



2.55-2.64 I 

 2.67-2.8 I 



(Quartz) 40.0 



Feldspar, \ 45.0 



l io f e t 97 Above 2<8 j Biotite j 



Epidote 3.9 'I Epidote, 1 



STRUCTURE. 



It is clear, from what has gone before, that both the Row- 

 landville and Port Deposit rocks accord in their geological 

 occurrence, as well as in their chemical and mineralogical 

 composition, with igneous granites. Both of them have, since 

 their solidification, undergone extensive alteration, but, as 

 has been already pointed out, this consists, in the Rowland- 

 ville area, mainly of chemical changes without much effect 



