QUARRY MATERIALS OF NEW YORK 33 



schistosity mark a weakened cohesion between the constituents 

 which may lead to a very considerable variation in their strength, 

 according as the latter is tested parallel with or normal to those 

 planes. Variations of strength do occur in the igneous rocks, 

 notably such as possess rift and grain structures, but to a minor 

 degree as compared with the other classes. 



Mineral composition affects the strength of rocks, though in 

 general it is less important than the features connected with texture. 

 Such a weak material as serpentine shows surprising compressive 

 and tensile strengths when the fibers of which it is composed are 

 thoroughly interwoven. Marbles and limestones of nearly uniform 

 composition exhibit a wide variation in tests with variations of 

 grain and compactness of texture. On the other hand, the presence 

 of hard resistant minerals like quartz, hornblende and pyroxene no 

 doubt contribute to the strength of certain igneous rocks. 



The resistance of the stone to stress necessarily differs with the 

 method of application, and the behavior of a sample under com- 

 pression, which is the usual method of testing strength, does not 

 afford any valuable information as to the resistance the stone will 

 offer to tensile or bending stresses. This fact is very well brought 

 out by the cracking of arches and lintels under transverse strains, 

 whereas the same forces applied in compression have little or no 

 effect. 



The strength of stone is often injured by lack of proper care in 

 quarrying. Stone that has been blasted from the ledge by dynamite 

 or powder can not be expected to exhibit the same strength as that 

 quarried with the use of the drill and wedges. Even if there are 

 no visible cracks or checks, it will be found that the blasting has 

 worked damage to the texture by loosening the bond between the 

 particles. 



Other conditions which affect strength are the weathering and 

 drying out of the stone after removal from the quarry. Some 

 soft sandstones show a remarkable gain in strength when exposed 

 to the sun's heat and the consequent evaporation oi the quarry sap. 

 When saturated again, they lose some of this acquired strength, 

 but are still more resistant than the freshly quarried rock ; exposure 

 to a wide range of temperature is, however, detrimental to any 

 stone. 



THE EXAMINATION AND TESTING OF STONE 



The availability of any stone for commercial use depends first 

 of all upon the features connected with its field occurrence. Geo- 

 logical observations are necessary to determine the quantity of 



