THE MINING AND QUARRY INDUSTRY I913 97 



lie interest in improved methods of highway construction. They are 

 now mainly employed in city streets, but they are well adapted for 

 any highways which bear a heavy traffic. Their cost at the quarries 

 is about $1.50 a square yard, or a little more than paving brick. 

 With the completion of the barge canal, which traverses the district 

 from east to west, the quarries will be able to reach a much larger 

 territory than heretofore. 



TRAP 



Trap is not a distinct rock type, but the name properly belongs 

 to the fine-grained, dark-colored igneous rocks that occur in in- 

 trusive sheets and dikes. In mineral composition it dii^ers from 

 most of the igneous rocks that are classed in the trade as granite 

 by the prevalence of the basic plagioclase feldspars and the higher 

 percentages of the iron magnesia minerals, while it contains no 

 quartz. Some of the so-called " black granites," however, are 

 trap. The name is sometimes applied to fine-grained rocks of 

 granitic or syenitic composition and sometimes even to rocks of 

 sedimentary derivation, but such usage is misleading and inde- 

 fensible. 



The particular value of trap is due to its hardness and toughness. 

 Its fine, compact, homogeneous texture gives it great wearing pow- 

 ers and it is eminently adapted for road metal and concrete of which 

 heavy service is required. The principal product, therefore, is 

 crushed stone. It has been used to some extent, also, as paving 

 blocks, but these are rather difficult to prepare, since trap very 

 seldom shows any capacity for parting comparable to the rift and 

 grain structures of granites. As a building stone it finds very little 

 application, probably on account of its somber color. The expense 

 of cutting and dressing trap is also an obstacle to its employment 

 for'building or ornamental purposes. 



The trap c[uarried in New York State is properly a diabase. Its 

 mineral composition varies somewhat in the different occurrences, 

 but the main ingredients are plagioclase, feldspar and pyroxene, 

 with more or less of amphibole, olivine, magnetite and some times 

 biotite. The texture is characteristic, for the feldspar forms lath- 

 shaped crystals which interlace and inclose the pyroxene and other 

 ingredients in the meshes, and it is this firmly knit fabric which 

 gives the stone the qualities of strength and toughness. 



The largest occurrence of trap in New York is represented by 

 the Palisades of the Hudson and the continuation of the same in- 



