LIMESTONE AND MARBLE 



423 



cent, of ferrous oxide and 7.36 per cent, of ferric oxide. Its 

 absorptive percentage was 0.15. It lost 0.07 per cent, in weight 

 in the sulphuric acid solution test. It remained unchanged in 

 tests of alternate freezing and thawing. 



The estimated production of red roofing slate in 1889 was 

 5,000 squares. The ruling prices per square were as follows :* 



Red $8 00 to $10 00 



Purple 3 50 to 4 00 



Unfading-green 3 50 to 4 00 



Sea-green , 2 75 to 3 00 



Variegated 2 50 to 2 75 



Note. — A recent bulletin of the United States Census gives a list of firms producing slate, 

 and the statistics of production, labor, wages, etc. According to this report there are sixteen 

 quarries in this State, which produced in 1889 17,167 squares of roofing slate, and slate for 

 other purposes valued at $44,877, maKing a total value of $130,603. 



LIMESTONE AND MARBLE 



Limestones consist essentially of calcium carbonate. They are, 

 however, often quite impure ; and the more common accessory 

 constituents are silica, clay, oxides of iron, magnesia, and bitumi- 

 nous matter. And these foreign materials may enter into their 

 composition to such an extent as to give character to the mass, 

 and hence they are said to be silicious, 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 of varieties. Thus, the magnesium carbonate may 

 be present, from traces, to the full percentage of a typical dolo- 

 mite. Or, the silica may range from the fractional percentage to 

 the extreme limit where the stone becomes a calcareous sand- 

 stone. Crystallized minerals, as mica, quartz, talc, serpentine 

 and others, also occur, particularly in the more crystalline 

 limestone. 



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 



* Letter of Hugh Williams of Middle Granville, January 22, 1890. 



