LIME AlsD CEMETntT INDUSTRIES 651 



MIJSTERALOGIC COMPOSITION' 



Pure limestone contains only carbonate of limCj and dolomite, 

 if pure, would contain only carbonate of lime and magnesia. 

 Impure limestones may contain many different mineral species. 

 The commonest of tliese is quartz, wliich may be present either 

 in the form of tiny grains or else as nodular masses (chert), 

 popularly known as flint. Clay is also a frequent impurity, and 

 iron oxid is common in some as a cementing material. 



The greatest variety of minerals is usually found in those 

 limestones which have been subjected to contact or regional meta- 

 m.orphism, as this causes a segregation and rearrangement of the 

 original impurities of the rock, yielding new mineral compounds. 

 Among the commoner minerals thus formed are pyroxene, am- 

 phibole, garnet, vesuvianite, epidote, zircon, wernerite, wol- 

 lastonite, graphite, etc. 



The many crystals of white pyroxene in the Westchester county 

 dolomites, and the bunches of dark minerals in the limestones 

 of the Adirondack region, and in those around Mt Adam, in 

 Orange county, are examples of this. 



In weathering the more silicious layers, or spots in the lime- 

 stone, stand out in relief on the weathered surface, so that this 

 often serves as a clue to the amount of mineral impurities which 

 the rock contains. 



Texture 



Limestones, unless metamorphosed, are commonly fine grained, 

 and may vary from fine earthy rocks to granular ones. Meta- 

 morphic limestones are often coarsely crystalline. 



The hardness of the rock (not of the individual grains) will 

 depend partly on the cementing material which binds the grains 

 together and partly on the shape of the grains themselves, whether 

 rounded or angular. The latter will have a tendency to inter- 



