Granites of Cecil County, in Xorth-eastern Maryland. 61 



appearance, which is made the more prominent by the pres- 

 ence of sharp, gable-like masses, which are due to the prob- 

 able existence of joint planes oblique to the foliation. At 

 this locality, a small ravine cuts through the cliff along the 

 strike of the rocks, excavating a deep channel; the vertical 

 walls along its sides, through gravity, bend over at their tops, 

 and so creep into the valley, giving the appearance of dip- 

 ping to the south at an angle of forty-five degrees. This 

 phenomenon is seen in Plate V, in which the observer is look- 

 ing toward the east. 



The smaller streams also yield good exposures, especially 

 near the river, because, as their sources are approached their 

 velocity is less and the channels become obstructed by gravel 

 and debris. The best exposures are found in the quarries 

 and railroad cuttings. It is of interest that the largest granite 

 quarries of Maryland are those just north of the town of 

 Port Deposit, operated at the present time by McClenahan & 

 Brother. Its economic value is seen in the fact that it has 

 been used in various government works, as in the forts along 

 the Chesapeake Bay and the Atlantic Coast, and in the founda- 

 tion of the Treasury building at Washington. 



The rock is of light color, with dark biotite arranged in 

 more or less parallel lines. This foliation has a north-east 

 direction at right angles to the river, while its dip is nearly 

 vertical. In addition to this parallel structure, there is 

 developed a system of joints which are of great aid to the 

 quarrymen in their work (see Plate VI). The principal joints 

 strike north-east, in the same direction as the foliation, but 

 their inclination to the parallel lines varies greatly even in a 

 short distance. A second set of vertical joints runs at right 

 angles to the main series, while a third set, trending west of 

 north, is inclined 60 degrees to the principal joints. A fourth 

 set of joints is approximately horizontal, but in some cases 

 these show a decided curvature. In a number of places there 

 are minor joints developed, which run at varying angles to 

 the main set, and cause the stone to break into small irregular 

 blocks. 



