MARYLAND GEOLOGICAL SURVEY 113 



These types, it is believed, represent differentiation of the magma 

 before intrusion, while their gradations represent subsequent differ- 

 entiation. 



PETEOGKAPHY OF THE CRYSTALLINE FOKMATIONS. 1 



Mica-Gneiss. 



The mica-gneiss does not possess a uniform lithologic character, 

 but is exceedingly heterogeneous. It is usually marked by an excess 

 of mica, but this is not always the case. When either muscovite or 

 biotite, or both micas, are conspicuous constituents, the rock is very 

 schistose and coarsely crystalline. 



As mica becomes less prominent, the formation becomes finer 

 grained, and either more gneissic or more quartzose. The amounts 

 of quartz and feldspar present vary considerably, but neither is 

 ever altogether absent. 



The most gneissic type is found in the exposure on the Susquehanna 

 at Bald Friar and northward. At Haines, just north of the State 

 line, the formation is very quartzose. Massive grey quartzose beds 

 alternate with thin chloritic beds, which show slip cleavage. In the 

 eastern part of Cecil county the formation is very micaceous. 



At Bald Friar the rock is fine-grained and a light shade of glisten- 

 ing greenish grey. 



In the hand specimen muscovite, quartz, feldspar and epidote can 

 be distinguished. The slides show abundant minute scales of mus- 

 covite and clear granulated quartz as the predominant constituents. 



Muscovite is the alteration product of rounded grains of an alkali 

 feldspar and also fills interstices. 



Quartz occurs in granulated lenses and veins. The former are 

 wrapped about with somewhat scanty green biotite. There is some 

 secondary epidote associated with the biotite. The slide has a clastic 

 and gneissoid character. The quartz lenses represent fractured 

 pebbles. 



On the road northwest from Oakwood, just north of Conowingo 



x For explanations of the more technical terms employed in the following descrip- 

 tions see the glossary at the end of this chapter. 

 8 



