118 THE CRYSTALLINE ROCKS OF CECIL COUNTY 



Undulatory extinction, polysynthetic twinning, the development 

 of microcline and partial granulation are further pressure effects. 

 Epidote is always present as an alteration product. It is sometimes 

 very abundant, and in well-defined crystals. This mineral can hardly 

 be considered a product of pure dynamic metamorphism. Its pro- 

 duction seems to belong to a zone intermediate between the zones of 

 metamorphism and of weathering. Like the products of the latter 

 zone it contains water and leads to the more ready disintegration of 

 the rock. While, like the products of the zone of metamorphism, it 

 forms in response to pressure and at great depth. More than one 

 species of feldspar is present. Orthoclase, which may show undula- 

 tory extinction or granulation, is most frequently altered to museovite. 



Microcline is recognized by the characteristic gridiron structure. 

 It may be somewhat granulated but is free from muscovitization. It 

 represents a molecular rearrangement of orthoclase in response to 

 pressure without an accompanying change in chemical composition. 



The predominating feldspar is plagioclastic and may constitute 

 about a third of the rock. It is an acid plagioclase. The extinction 

 angles place it between Ab 3 An x and Ab 2 An 1? it accordingly corre- 

 sponds to the oligoclase species. The feldspars often show a marked 

 tendency towards idiomorphism, while zonal structure is common, 

 though somewhat obscured by the alteration products, 



CHEMICAL COMPOSITION. 



The following chemical analysis of the Port Deposit granite was 

 made for Dr. Grimsley by the late Win. Bromwell, of Port Deposit, 

 at the laboratory of the Johns Hopkins University. 



Si0 2 73.69 



A1 2 3 12.89 



Fe 2 3 1.02 



FeO 2.58 



MgO 50 



CaO 3.74 



Na 2 .. 2.81 



K a O 1.48 



H 2 1.06 



Total 99.77 



