78 



Cincinnati Society of Natural History. 



THE GRANITES OF CECIL COUNTY, IN NORTH- 

 EASTERN MARYLAND. 



By G. P?;rrv Grimsley. 

 II. 



Microscopical Description of the Granites of 



THE RoWIvANDVII^LE AREA. 



When the microscope is applied to the study of the 

 granites whose macroscopic characters have been outlined, 

 much additional light is thrown on their origin and history. 

 While these rocks have not been so much altered as to wholly 

 destroy their original features, they nevertheless display in 

 an instructive manner the effects of dynamic action which 

 has changed both their constituents and structures. Old 

 minerals have been more or less metamorphosed, new ones 

 have been developed, and a secondary foliated structure has 

 been produced. 



In the Rowlandville region these effects are less intense 

 than in the vicinity of Port Deposit. While some crushing 

 and secondary foliation has here resulted, the alteration is 

 more mineralogical than structural. In the present chapter, 

 therefore, we have, after reconstructing from what now 

 remains the original rock type, to trace out more especially 

 the development of new minerals, while in the succeeding 

 chapter, on the Port Deposit granite, structural changes will 

 become more prominent. 



The most conspicuous metamorphic change in the Row- 

 landville rocks is the very extensive development of epidote. 

 Physical conditions seem to have been most favorable to the 

 production of this compound. It appears everywhere as a 

 new metamorphic product, and exhibits a variety of appear- 

 ances. It forms well-defined crystals, rounded grains and 

 hair-like needles in all the original constituents alike. As a 

 principal result of metamorphism in the Rowlandville area, 

 this mineral will demand a good share of our attention in the 

 present chapter. 



