GEOLOGICAL POSITION AND GEOGRAPHICAL DISTRIBUTION 207 



buildings in Jersey City and Hoboken are built of it. 

 There is a large quarry on the river at Rockland lake, the 

 output of which is for street work and road material almost 

 exclusively. 



Limestones and Marbles 



Limestones consist essentially of calcium carbonate. 

 They are however often quite impure ; and the more com- 

 mon accessory constituents are silica, clay, oxides of iron, 

 magnesia, and bituminous matter. And these foreign ma- 

 terials may enter into their composition to such an extent 

 as to give character to the mass, and hence they are said to 

 be siliceous, argillaceous, ferruginous, magnesian, dolomitic, 

 and bituminous. 



The chemical composition is subject to great variation, 

 and there is an almost endless series of gradation between 

 these various kinds or varieties. Thus, the magnesium 

 carbonate may be present, from traces, to the full percentage 

 of a typical dolomite. Or, the silica may range from the 

 fractional percentage to the extreme limit where the stone 

 becomes a calcareous sandstone. Crystallized minerals, as 

 mica, quartz, talc, serpentine and others, also occur, particu- 

 larly in the more crystalline limestones. 



In color there is a wide variation — from the white of the 

 more nearly pure carbonate of lime through gray, blue, 

 yellow, red, brown, and to black. The color is dependent 

 upon the impurities. 



The texture also, varies greatly. All limestones exhibit 

 a crystalline structure under the microscope, but to the 

 unaided eye there are crystalline and massive varieties. 

 And there are coarse-crystalline, fine-crystalline, and, sub- 

 crystalline, according as the crystals are larger, smaller, 

 or recognized by the aid of a magnifying glass only. The 

 terms coarse-grained and fine-grained may apply when there 

 is a resemblance to sandstone in the granular state of aggre- 

 gatlon. Other terms, as saccharoidal (like sugar), oolitic, 

 3 



