Granites of Cecil County, in North-eastern Maryland. 91 



Microscopical Description of the Rocks of the 



Port Deposit Area. 



III. 



In chapter one, it was suggested that the staurolitic mica- 

 sehist, just south of Liberty Grove, be used as a division line 

 in the granite area described in this paper. The rocks of the 

 district north of this belt have been considered in the pre- 

 ceding chapter. The petrographical characters of the rocks 

 in the Port Deposit area, or the region south of this zone, 

 will now be given. The metamorphism in the northern area, 

 as we have seen, is characterized by chemical change, while 

 the rocks show but little structural alteration. The granites 

 of the Port Deposit area, on the other hand, are characterized 

 rather by structural than by chemical changes. 



There are two main rock types in the Port Deposit area : 

 the granite, or, more properly, the granite-gneiss, and the 

 diorite, which occurs in narrow belts within the more acid rock. 



GRANITE GNEISS. 



This rock is of such economic importance that it calls for 

 special attention. As to the proper designation of the Port 

 Deposit rock there has been considerable difference of 

 opinion, as it has been called a syenite, granite and gneiss. 

 The presence of much quartz and the very small amount of 

 hornblende which it contains show that it can not with pro- 

 priety be called a syenite, either in old or new acceptation of 

 that term. The word gneiss has been used in a very indefinite 

 way, so that it is desirable to clearly define the term. In this 

 paper the word gneiss will be used in a purely structural 

 sense, to denote all feldspathic crystalline schists.* If the 

 origin of such metamorphic feldspathic schists can not be 

 determined, the) 7 are called simply gneiss ; but if their origin 



:; The word schist is here used to include all holocrystalline rocks with a pro- 

 nounced parallel or foliated structure. See A. Geikie, Text-Book of Geology, 3d 

 Edition, p. 178. 



