

126 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



feldspar present varies widely, from nearly zero in certain facies to 

 more than 50 per cent of the rock in the normal type. It is gabbro- 

 itic in the manner of its occurrence, in broad, irregular areas, seldom 

 exhibiting crystal boundaries. Both in the norites and in the hyper- 

 sthene-gabbro the feldspar is often sanssnritized. This change is 

 associated with the passage of the pyroxene, whether monoclinic or 

 orthorhombic, into hornblende. Chlorite is a less common alteration 

 product of the feldspar. 



The pyroxenic constituent is sometimes wholly an orthorhombic 

 variety hypersthene. This mineral possesses the usual gabbroitic 

 allotrimorphic character, and exhibits the brilliant trichroism and 

 optical orientation of hypersthene. 



A monoclinic pyroxene, with a pinacoidal parting in addition to 

 the prismatic cleavage, usually accompanies the hypersthene and 

 is, in the hypersthene-gabbro, nearly equal or more rarely predomi- 

 nating in amount. 



Pyroxene, like the feldspar, varies in proportion. It is rarely less 

 than two-fifths of the rock in which it occurs and may constitute a 

 much larger percentage. 



Hornblende and chlorite are its alteration products. The former 

 mineral sometimes entirely replaces the pyroxene. 



Quartz is an original constituent of the norite as of the hornblende- 

 gabbro, but is much more rarely found. 



Olivine is also rarely found. It occurs in a hypersthene-gabbro 

 associated with peridotite, one-quarter of a mile north of Harrisville, 

 near Oak Grove schoolhouse. 



Olivine, whenever it comes in contact with the feldspar (bytown- 

 ite), is surrounded by a double reactionary rim. The inner border 

 is composed of a colorless highly polarizing mineral with a consider- 

 able index of refraction. The outer border is rendered almost 

 opaque by the close aggregation of a green, fibrous mineral. The 

 index of refraction and the polarization colors are high in this case 

 also. 



