Granites of Cecil County, in North-eastern Maryland. 79 



The development of secondary muscovite is less important 

 in this region, where the strain and crushing effects are slight, 

 compared with those in the Port Deposit granites. 



Reconstruction of the Rowlandville Granite Type. 



We can readily reconstruct the Rowlandville granite type 

 as it was before it suffered any metamorphism. It was a 

 normal granitite or biotite granite of usual hypidiomorphic 

 structure, and containing a large proportion of plagioclase of 

 the albite-oligoclase series. In places it may have contained 

 some original muscovite. It presented few structural varia- 

 tions, but showed a much more basic facies toward the 

 adjoining gabbro. 



original essential components. 



The original essential constituents of the Rowlandville 

 granites are feldspar, biotite, muscovite and quartz. Though 

 orthoclase is found in small amount, the prevailing feldspar 

 belongs to the more acid portion of the plagioclase series, 

 albite and oligoclase, as determined by extinction angles and 

 specific gravity. These feldspars are usually free from in- 

 clusions, and often possess a well-marked zonal structure, 

 though in many cases this is partially or completely obliter- 

 ated by metamorphic changes. They twin usually according 

 to the Carlsbad law, though Baveno and Roc Tourne types 

 also occur. 



The biotite is less in amount than the feldspar, and is 

 strongly pleochroic, with dark brown' color in the direction 

 of the cleavage, and light yellow at right angles to these lines. 

 It occurs in basal and prismatic sections, but seldom in very 

 large plates, since it is more or less broken. Its optical angle 

 is small, the dark cross opening slightly, and its extinction is 

 nearly parallel. It contains numerous inclusions of zircon, 

 with pleochroic halos, sphene and epidote. A very character- 

 istic feature in many sections is the presence of numerous 

 needles crossing each other at 6o° angles, which will be more 

 fully described in another place. While muscovite is abundant 



