176 NEW YORK STATE MUSEUM 



Section 6 



THE NEW YORK MARBLE QUARRIES 

 GENERAL CHARACTERS OF MARBLES 



Marble, like granite, is a term used by quarrymen for a variety 

 of rock materials. Any limestone that takes a polish or possesses 

 ornamental qualities is a marble in the trade sense, and some of 

 the softer silicate rocks are likewise thus designated, notably those 

 having a serpentine base. More properly the name belongs to the 

 crystalline or metamorphic class of limestones as distinguished 

 from the compact to finely granular kinds occurring in the regularly 

 bedded formations. 



The quality of crystallinity is not always lacking in ordinary 

 limestones, for some show aggregates of plainly visible calcite grains 

 with the characteristic calcite cleavage surfaces ; for example, the 

 Chazy limestones of the Champlain valley. But their texture is 

 never so completely crystalline as in the types that have undergone 

 a metamorphic rearrangement of their constituents while subjected 

 to compression in the depths of the earth. Such partially crystalline 

 limestones often polish well, but lack the glint and translucency 

 of true marbles. In this case, the presence of coarse crystalline cal- 

 cite probably results from the working over of the finely divided 

 particles by ground waters. 



The microscopic appearance of a true marble is quite distinct 

 from that of any carbonate rock which has not undergone pressure 

 metamorphism. In the first place, the particles of calcite (or 

 dolomite) are more uniform as to size and shape, whereas the 

 texture of nonmetamorphic limestones is apt to be very variable and 

 the size of grain shows a wide range. When crystallization takes 

 place under conditions of cubic compression which characterizes 

 the metamorphic process at considerable depths, the individual par- 

 ticles have not opportunity to develop the characteristic outward 

 forms that calcite ordinarily assumes, but must accommodate them- 

 selves to the narrow space restrictions resulting from the simul- 

 taneous crystallization of the whole mass. As a consequence, they 

 exhibit a more or less even, granular habit with curved or irregular 

 boundaries which are closely matched together. A second charac- 

 teristic of the metamorphic liinestones as seen in thin section is the 

 striations, broader than the lines of cleavage, that cut across the 

 grains. These mark the junctions of crystals in so-called twinned 



