Granites of Cecil County, in North-eastern Maryland. 89 



An attempt was made to calculate the proportionate min- 

 eral composition of this rock with the aid of what could be 

 learned from its microscopical examination. Such a calcula- 

 tion could only be an approximation, for two reasons ; First, 

 because the exact composition of the individual minerals in 

 the rock is not known. Second, because certain of the bases 

 enter into two or more of the silicates. Although a little 

 secondary muscovite occurs in the feldspar, this was ignored 

 and the proportion of orthoclase was first calculated, assum- 

 ing that all the potash was contained in this mineral. The 

 soda was, in like manner, referred entirely to the albite 

 molecule in the plagioclase. The magnesia was regarded as 

 being confined entirely to the biotite. A small proportion of 

 the lime (1%) was arbitrarily assumed to represent approxi- 

 mately the proportion of the anorthite molecule, while the 

 remainder, with the exception of what was necessary for the 

 sphene and apatite, was referred to the epidote. While some 

 titanium may be present in the biotite, this element was 

 assigned entirely to sphene, which was the only titanium 

 mineral shown in the thin section. The residual silica repre- 

 sents the quartz, and the residual iron the magnetite. 



For those minerals which have a definite composition like 

 orthoclase, albite, anorthite, quartz, sphene, apatite and mag- 

 netite, the theoretical proportions of the constituents were 

 used as given in E. S. Dana's System of Mineralogy. On 

 account of the almost colorless character of the epidote, this 

 mineral was assumed to be composed of the iron and alumina 

 molecules in the ratio of 1 to 8. 



The results of this calculation are given in the following 

 table : 







Apatite, 



25 " " 





35 " " 



Biotite, 



14.00 " 





12.25 " " 



Albite, 





Anorthite, 



5.00 " 







Epidote, 



: 14 25' " .^ 



Total, 



