QUARRY MATERIALS OF NEW YORK 17 



stress. Thus two joint systems arise from a single force and even 

 more complex fractures may result, as has been demonstrated by 

 Becker. 1 



In most rock exposures there are at least two systems of vertical 

 or highly inclined planes nearly at right angles, and one that lies 

 approximately horizontal, or in the sedimentary and metamorphic 

 rocks, follows the bedding or schistosity. 



The joints in one direction may be more clearly marked and per- 

 sistent than in other directions. They can be divided into principal 

 joints and minor joints. The latter often originate and die out in 

 a short distance, but the major joints are likely to continue over 

 wide areas. Within the crystallines of the Adirondacks, the most 

 persistent joints have a northerly to northeasterly trend with a 

 complementary set at right angles. 



A series of closely spaced vertical joints is known to quarrymen 

 as a heading. The zone of broken rock is used as a back or heading 

 to work against. In such close jointing, there is often evidence of 

 more or less faulting in the smoothed and striated surfaces and 

 the formation of secondary minerals. A weathered appearance is 

 also characteristic of such zones, as they serve as channels for the 

 admission of surface waters. 



The igneous rocks, especially those like granite that occur in 

 bosses and knobs, show at times a series of close-set fractures, 

 horizontal or slightly curved in conformity with the surface, that 

 divide the mass into parallel plates. This is known as sheet struc- 

 ture and is common in many of the New England and southern 

 granites, but appears to be rare in the Adirondacks, at least in its 

 more typical form, although some quarries show incipient or im- 

 perfectly developed sheets. The origin of this structure has re- 

 ceived much attention from geologists, with the proposal of various 

 explanations. Since the fractures follow the surface contours in 

 most cases and gradually diminish in their frequency and strength 

 with depth, there seems to be good reason for connecting them with 

 some superficial process like the strains set up by temperature varia- 

 tions. The subject is well discussed in Dale's reports on the quar- 

 ries of the New England States. 2 



Faults. The phenomena incident to displacements of the rocks 

 along fractures are quite common in the crystalline areas, and also 



1 Proceed. Washington Acad. Sci., v. 7, July 1905, p. 267-75. 



2 For example, " The Chief Commercial Granites of Massachusetts, New 

 Hampshire and Rhode Island." U. S. Geol. Sur. Bui. 354, 1908, p. 22-29. 



