112 



THE CRYSTALLINE ROCKS OF CECIL COUNTY 





Silica. Specific Gravity. 





Granite. Segregation. Granite. 



Segregation. 



Port Deposit, Maryland. % 



73.7 62.2 2.69 2 83 



Peterhead, Scotland 



Snap Fell, Westmoreland 



73.70 61.39 2.69 

 69.78 56.95 2.69 

 69.64 65.01 2.72 

 68.97 57.89 2.68 



2.73 



2.77 



Greadv, Cormvall 



2 73 



Barr-Andlau, Alsace 



2.78 



\Yhile, therefore, these basic segregations cannot be considered a 

 proof of eruptive contact between granite and gabbro, gabbro dikes 

 in the neighborhood both of the northern and southern gabbro belt 

 are considered contemporaneous intrusions of the same magma, and 

 proof of an intrusive origin for the gabbro, subsequent to the granite. 



The relation of the norite to the pyroxenites and peridotites is more 

 distinctly expressed. Between their alteration product, serpentine, 

 and the norite can be drawn a distinct line, and in the norite occur 

 pyroxenite and peridotite dikes. The triangular area of serpentine 

 at the head of Xortheast Creek, on the State line, is an apophysis 

 from the " State line serpentines." 



Between this apophysis and Sylmar there is exposed an oval mass 

 of serpentine which touches the State line, but lies mainly in Penn- 

 sylvania. Southwest of Sylmar and just south of the junction of 

 the Rising Sun and Calvert roads (by Mount Hope Church), there 

 is an oval mass of serpentine and peridotite, somewhat more than a 

 half mile long and nearly one quarter of a mile wide. Two miles 

 west of this locality and two miles north of Harrisville, is exposed 

 another included mass of meta-peridotite. Notwithstanding the ap- 

 parent sharpness of the boundary between meta-peridotite or meta- 

 pyroxenite and norite, there are intermediate types along the border, 

 norites in which the feldspar is reduced and pyroxenites containing 

 feldspar. 



There are, in short, four igneous types amid all these gradations. 



An acid feldspathic type; an intermediate feldspathic type; a basic 

 feldspathic type and a non-feldspathic type. 



