ECONOMIC GEOLOGY 197 



nous matter. These foreign materials may enter into their com- 

 position 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. Thus, the magnesium carbonate may vary in 

 quantity from a trace, to the full percentage of a typical dolomite. 

 Or, the silica may range from a fractional percentage to the ex- 

 treme limit where the stone becomes a calcareous sandstone. 

 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 impuri- 

 ties. 



The texture also varies greatly. All limestones exhibit a crys- 

 talline structure under the microscope, but to the unaided eye 

 there are crystalline and massive varieties. There are coarse 

 crystalline, fine crystalline, and sub-crystalline varieties, accord- 

 ing 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 aggregation. Other terms, as saccharoidal 

 (like sugar), oolitic, when the mass resembles the roe of a fish; 

 crinoidal, made up of the stems of fossil crinoids, also are in use, 

 and are descriptive of texture. The state of aggregation of the 

 constituent particles varies greatly, and the stone is hard and 

 compact, almost like chert, or is loosely held together and crum- 

 bles on slight pressure, or again it is dull and earthy as in chalk. 

 The crystalline, granular limestones, which are susceptible of 

 a fine polish, and which are adapted to decorative work, are 

 classed as marUes. Inasmuch as the distinction is in part based 

 upon the use, it is not sharply defined and scientific. Generally 

 the term is restricted to those limestones in which the sediments 

 have been altered and so metamorphosed as to have a more or 



