QUARRY MATERIALS OF NEW YORK 63 



foot. In fresh condition they are fairly hard and exceedingly 

 tough, but lose these qualities rapidly if decomposed by atmospheric 

 weathering. Their decomposition is sometimes hastened by the 

 presence of sulphides, which are likely to be abundant in places, 

 more so than in acid rocks. The characteristic alteration product of 

 the more basic gabbros is serpentine. 



Gabbros find little employment for architectural work, owing to 

 their somber appearance. They are used to some extent for dec- 

 orative and monumental purposes under the trade name of " black 

 granite." Quarries in Maine, Minnesota and North Carolina have 

 supplied such stone, but very little has come from the large gabbro 

 areas of New York. The main developments in this State have 

 been for the supply of crushed stone for macadam and concrete, for 

 which purposes the fine-grained dense sorts may be considered equal 

 to the best trap. 



The limited use of the stone for general purposes is partly due, 

 no doubt, to the expense of dressing it. The basic rocks seldom 

 show any rift or grain structure, but break with a curved fracture 

 without reference to direction. 



DIABASE OR TRAP 



Trap is a popular term for the dark, fine-grained igneous rocks 

 that occur in intrusive sheets and dikes. It is thus not a distinct, 

 rock type, but may include diabase, basalt and any of the basic 

 intrusions which have a sheetlike form. In New York State, the 

 name is equivalent practically to diabase, an intrusive containing 

 lime-soda feldspar and pyroxene as essential ingredients, with 

 subordinate amphibole, olivine and pyroxene. The composition 

 thus is very similar to that of gabbro, but the appearance of the 

 rock is quite characteristic, owing to the manner in which the 

 minerals are distributed. The feldspar forms laths or rectangular 

 rods that inclose the pyroxene, olivine and amphibole in their ir- 

 regular interspaces like a network. This gives a firmly interlocked 

 texture which insures a high degree of toughness and resistance to 

 abrasion. 



Diabase is almost black on rock face and polished surfaces. Like 

 gabbros, it finds limited employment for structural stone. Its 

 specific gravity is about 2.9 and the weight around 180 pounds to 

 the cubic foot. Its fine grain promotes evenness of wear, so that 

 with its other qualities it is exceptionally well adapted for road 

 material and concrete in all cases that involve heavy duty. Some 

 examples make a good black granite, as shown by specimens of 



