QUARRY MATERIALS OF NEW YORK 21 



from compression when once they have consolidated. The very 

 early gneisses of probably igneous derivation have a lenticular or 

 belt-like form with the large axis parallel to the general structural 

 trend and have thus been influenced to some extent, though they 

 were perhaps squeezed out somewhat while still molten. In general, 

 the ligneous masses serve as a buttress, against which the thrusts that 

 fold the sedimentaries have little effect. 



DIFFERENTIAL PARTING 



Many rocks as found in the field show a capacity for splitting 

 along one or more planes. This feature, when well developed, is 

 of great advanltage tO' the quarryman and stone dresser and upon 

 its existence depends in great measure the availability of stone 

 for many commercial uses. There is naturally marked variation in 

 the behavior of rocks in regard to parting, not only between the 

 different classes — igneous, sedimentary and metamorphic — but 

 also among members of the same class, so that each occurrence must 

 be separately tested for this structure. 



In the sedimentary class, the direction of easiest parting coin- 

 cides usually with the bedding. In the finer mechanical sediments 

 like bluestone and shale that have been sorted and deposited by 

 water, the structure is often exhibited in great perfection. In this 

 case, it may be traced to the presence of platy and elongated par- 

 ticles among the constituents, or else to a regular alternation of 

 finer and coarser materials parallel with the bedding planes. The 

 chemical precipitates, which are mainly represented by limestones, 

 show it much less frequently, being often incapable of smooth frac- 

 ture, although subdivided by natural seams or joints. For that 

 reason limestones are often stronger and more resistant to wear 

 than the other sediments, and are specially adapted for crushed stone 

 in road-making and concrete. 



The best example of this parting among sedimentary rocks is 

 found perhaps in the flagstones which are mostly made of fine- 

 grained sandstones that in New York are abundant in the Devonic 

 formations. They are locally known as bluestone, though, that 

 term is not always expressive of their appearance. Between the 

 bedding planes of the sandstones occur closed seams which are in- 

 dicated by a slight change of grain and are spoken of by the 

 quarrymen as " reeds." According to Dickinson,^ reeding quarries 

 are found generally in the fine-grained stone and each locality or 



^ Quarries of Bluestone and Other Sandstones. N. Y. State Mus. Bui. 6i, 

 p. y-8, 1903. 



