JO NEW YORK STATE MUSEUM 



eastern side, but have little importance elsewhere. The ophitic lime- 

 stones that have been quarried at different times belong to the same 

 series. The marbles of the Adirondacks comprise both the calcite 

 class with very little magnesia and the dolomite class containing 

 high percentages of magnesia. No definite relation is apparent in 

 regard to the occurrence of the two and both may be found in the 

 same area and in close association. 



The southeastern New York marbles occur in belts which follow 

 the north-south valleys, east of the Hudson, from Manhattan island 

 into Westchester, Dutchess and Columbia counties. They range 

 from very coarsely crystalline to finely crystalline rocks, are pre- 

 vailingly white in color and belong to the dolomite class. They are 

 interfolded with schists and quartzites, the whole series having 

 steep dips like those of strongly compressed strata. The geologic 

 age of the southern belts is probably Precambric, but on the north 

 and east within range of the Taconic disturbance, they may belong 

 to the early Paleozoic. 



Bodies of practically pure serpentine of considerable extent are 

 found on Staten Island and in Westchester county near Rye ; they 

 represent intrusions of basic igneous rocks whose minerals, chiefly 

 pyroxene and olivine, have subsequently changed to serpentine. 

 They are not important for quarry purposes, owing to the fre- 

 quency of fissures and joints and the rather somber color of the 

 exposed part of the masses. 



The microcrystalline or subcrystalline limestones that are Some- 

 times sold as marbles include members of the regularly bedded 

 unmetamorphosed Paleozoic limestones, which locally show quali- 

 ties of color and polish that make them desirable for decorative 

 purposes. They range from dense granular varieties to those hav- 

 ing a more or less well-developed crystalline texture and are often 

 fossiliferous. Inasmuch as they have never been subjected to 

 regional compression or been buried in the earth deep enough to 

 become heated, the crystalline texture, when present, may be 

 ascribed to the work of ground waters. These circulate through 

 the mass, taking the carbonates of lime and magnesia into solution, 

 and redeposit them in crystalline form. Originally, the limestones 

 were accumulations of lime-secreting fossils or granular precipitates, 

 for the most part of marine origin. Some of the localities where 

 these unmetamorphic marbles occur are on the west shore of Lake 

 Champlain, around Plattsburg and Chazy (Chazy limestone), Glens 

 Falls (Trenton limestone) and Becraft and Catskill (Becraft lime- 

 stone). 



