Niles.] 82 [January 4, 



thus breaking the rock into slabs of regular form. The beds are so 

 free from seams, and the rock is so homogeneous in structure, that 

 pieces of great length may be obtained in this manner. In working 

 from the lower side of the quarry towards the upper, the edges of 

 those portions of the beds which are still in position, rise above each, 

 other like successive steps. The edges of these steps form nearly 

 straight lines, parallel to the strike of the strata. When these por- 

 tions of the beds remain sometime without being worked, cracks 

 frequently appear along these edges. If still allowed to remain 

 undisturbed, the fractures become more extensive, and frequently 

 break up large quantities of the excellent stock of the bed. It 

 has been found by experience that when these cracks first appear 

 their extension may be prevented, and the stock preserved from fur- 

 ther injury, by making an opening in the bed, trending in easterly 

 and westerly directions, thus cutting it across the strike, and at 

 right angles to the ordinary working lines. It is evident that such 

 an east and west opening could counteract only a force which is 

 exerted in nearly north and south directions. That the force works 

 in the direction of the strike of the strata, is still further evident upon 

 the examination of some of these fissures. A single example may be 

 sufficient for illustration. 



When at the- quarry, April 29th, 1871, I observed an irregular 

 fracture in a bed three feet and nine inches in thickness. The frac- 

 ture was about sixty-one feet long. The northern portion of thirty- 

 eight feet was nearly parallel to the strike, and the southern part of 

 twenty-three feet curved to the eastward, and was accompanied by 

 secondary fractures near it. Commencing at the northern end of the 

 fracture and going southward, it would be noticed that at a few 

 places it turned suddenly to the east, and immediately to the south 

 again, thus making two right angles in its course. At these places 

 the very short portions of the fracture trending eastward were wider 

 than the long ones running southward, thus showing that the great- 

 est movement had been from the north towards the south. At 

 that time, April 29th, the southward movement had amounted to 

 three-sixteenths of an inch ; but when I visited the quarry July 7th, 

 it had amounted to five-eighths of an inch, though at the time, the 

 greatest width of the prack, at any place where it followed its general 

 north and south course, was only one-fourth of an inch. Therefore 

 the force which fractured the rock, hads moved the relieved portion 

 southward, more by three-eighths of an inch than it had been moved 

 westward. 



