198 THE MINERAL RESOURCES OF CECIL COUNTY 



of joints runs almost at right angles to the first and is almost as sharp 

 as those of the main series. A third set trending west of north is 

 inclined 60° to the principal joints, while a fourth set, approximately 

 horizontal, serves as bedding joints. The surface of the jointing 

 plane is usually quite smooth and even, but the direction and distance 

 between the parallel surfaces is not always constant. This produces 

 a slight wedging in the blocks, which increases somewhat the cost of 

 quarrying. On the other hand the smoothness of the joint surface 

 frequently renders the rock ready for use in building without the 

 intervention of the stone cutter, and allows the extraction of enor- 

 mous nearly rectangular blocks. The expenses of preparing the rock 

 for use in the wall is accordingly reduced. 



Although there are some half dozen series of jointing the rock a 

 short distance below the surface is very compact, homogeneous, and 

 strong, as is shown by the pressure tests of Gillmore, who found that 

 the compressive strength of this rock was 13,100 pounds per square 

 inch when tested " on edge," and still more clearly by the more re- 

 cent tests 1 which show a crushing strength of over 80,000 pounds on 

 two inch cubes. The incipient jointing planes, although so closely 

 welded together as to show this great strength, are made use of by 

 the quarrymen in trimming the huge monoliths and in cutting the 

 smaller Belgian paving blocks, as the rock may be readily opened by 

 means of wedge and " feathers!" 



The distance between the major joints, which varies from half an 

 inch to several feet, is sufficiently great to allow the extraction of 

 any sized block, which can be handled advantageously by the ma- 

 chinery and by the transporting agencies. It is usually considered 

 that the rock of the Port Deposit quarries is somewhat more easily 

 worked than that at Frenchtown, which is otherwise indistinguish- 

 able. This difference in working arises in part no doubt from the 

 greater age of the quarries, better facilities for quarrying and hand- 

 ling, and also from the more convenient position of dominant lines of 

 working in the Port Deposit quarries. 



The texture of the Port Deposit granite, or granite-gneiss is highly 

 characteristic. The rock is composed of the usual granitic constitu- 



1 Edward B. Mathews, Maryland Building Stones, Md. Geol. Survey, vol. ii, 1898, 

 pp. 144-145. 



