178 NEW YORK STATE MUSEUM 



no discernible difference in the outward appearance of a calcite 

 limestone and a dolomite, and their distinction requires the use of 

 chemical or microscopic methods. The slight difference in hard- 

 ness is not a reliable criterion. The two minerals have similar 

 crystal properties, including perfect cleavage which yields surfaces 

 of rhombic outline. It is this cleavage that produces the bright 

 reflections of light and gives life to the crystalline marbles. 



The impurities in marbles take the form usually of scattered 

 grains or crystals of the same order of magnitude as the calcite 

 particles. In bedded limestones, on the other hand, they are dis- 

 tributed more or less evenly through the mass and consist of finely 

 divided clayey and siliceous materials — the mechanical sediment 

 formed during the deposition of the dissolved carbonates. The 

 clay and silica form new combinations in the process of meta- 

 morphism, the carbonates supplying the lime and magnesia that 

 may be required for the secondary minerals. Among the common 

 foreign ingredients are muscovite, diopside and tremolite, but a 

 great number of other silicates may occur. Any fine carbon is 

 converted into scaly graphite. Some of the silica may remain as 

 quartz. The iron minerals include hematite, magnetite and pyrite. 

 The last-named is most harmful if present in any amount, since 

 it decomposes readily in the atmosphere, producing a rusty stain 

 which will spread over large areas. 



TEXTURE 



The texture of marbles varies greatly between examples from 

 different localities. Some characteristic textures of New York 

 marbles are illustrated in figures 15 and 17. The grain may be 

 medium or fine, or may be uneven through the occurrence of differ- 

 ent sizes of particles. The shape and arrangement of the particles 

 also are quite variable and upon these features depend to a great 

 extent the strength and weathering qualities of the stone. The 

 Gouverneur monumental marble, composed predominately of calcite, 

 has a very compact texture, with grains of uneven size and of 

 angular to subrounded form. The particles frequently show dentate 

 outlines by which they are firmly interlocked; the general appear- 

 ance in fact is suggestive of the welded and dovetailed arrange- 

 ment exhibited by some granites. The dolomite marbles of south- 

 eastern New York range from exceedingly coarse to very fine- 

 grained varieties, but usually the grain in any one sample is fairly 

 even. Some have a compact and firmly knit texture and then are 



