186 NEW YORK STATE MUSEUM 



oxides; some of the rich purple tints are said to be due to oxide 

 of manganese. : 



The gray and blue tints are produced by iron in the form of 

 ferrous silicate or carbonate. By an irregular association of 

 masses of different colors a variegated surface is produced, or by 

 an alternation of white and variously-colored laminae a striped 

 appearance is given to the mass. 



Sandstones occur stratified and in beds of greater or less thick- 

 ness, and they are said to be thick-bedded or thin-bedded. In 

 some cases the beds are so thick, and the stone of such a uniform 

 texture, that the stone can be worked equally well in all direc- 

 tions, and is known as freestone. A laminated structure is com- 

 mon, especially in the thin strata, or when the stone is micaceous. 

 When the beds can be split into thin slabs along planes parallel 

 to the bedding, it is called a flagstone. A less common structure 

 is what is termed lenticular or wedge-shaped, in which the upper 

 and under surfaces lack parallelism, and the beds wedge out. It 

 makes the quarrying more difficult, and produces more waste 

 material. 



The variations in the nature of the component grains, and 

 binding material, in their arrangement, and in the forms of bed- 

 ' ding, produce a great variety of stone, and the gradations from 

 one to another are slight. The hardness, strength, beauty and 

 durability are determined by these varying elements of constitu- 

 tion. The stone best resisting the action of the atmospheric 

 agencies is that in which the quartz grains are cemented by a 

 silicious paste, or in which the close-grained mass approaches 

 in texture a quartzite. 



The presence of mineral liable to decomposition, as feldspar 

 highly kaolinized, of mica, marcasite, and pyrite, of calcite in 

 quantity, and clays, affects the durability and tends to its de- 

 struction, i 



Sandstones are classified according to their geologic age also. 

 They are found occurring in all the series, from the oldest to the 

 most recent formations. Those of a given age are generally 



