64 Cincinnati Society of Natural History. 



inclusions of a foreign rock; and second, that they are segre- 

 gations of the more basic constituents of the granite, formed 

 before its solidification. The facts observed within the 

 region studied seem, on the whole, to indicate that these 

 patches are basic segregations in the granite. The only other 

 rocks which are certainly known to occur near at hand, and 

 which might have furnished material for inclusions, are the 

 gabbro and staurolite schist. There is, however, nothing to 

 suggest gabbro or staurolite schist fragments, either macro- 

 scopically or in thin sections. Their mineralogical composi- 

 tion is, in all cases studied, the same as that of the granite 

 in which they occur, except that they are richer in ferro- 

 magnesian constituents. 



To test the relative acidity of the dark patches and the 

 granite in which they occur, determinations of silica and 

 specific gravity were made for me by Messrs. Reid and 

 Magruder in the chemical laboratory of the University. In 

 the following table the result is given, together with deter- 

 minations of similar material from other regions.* 





Silica. 



Specific Gravity. 



Granite. 



Segrega- 

 tion. 



Granite. 



Segrega- 

 tion. 





73-7 



62 . 2 



2.69 



2.83 





73-7 



64-39 



2.69 



2-73 





69.8 



56.95 



2.69 



2.77 



Barr, Andlau, 



68.9 



57-89 



2.68 



2.78 



Gready, Cornwall, 



68 9 



57.89 



2.68 



2.78 



Most observers of similar patches in other localities have, 

 after careful study, reached the same conclusion with regard 

 to their origin. References to such work are given by 

 Phillips, f Hatch, J and Zirkel|| in their papers on dark patches 

 in granite. 



While the weight of evidence indicates that the dark 

 patches which are sparsely disseminated through the Row- 

 landville and Port Deposit granites at the localities above 



*Zirkel Lehrbueh der Petrographie, Vol. I, p. 789, 1893. 

 fQuart. Journ. Geol. Soe. (London), Vol. 36, p. i, 1880. 

 \ Quart. Journ. Geol. Soc. (London ), Vol. 44, p. 548, 1888. 

 || loc cit. 



