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NEW YORK PTATE MUSEUM 



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. When finegrained it is 

 often designated as liver-rock. A laminated structure is 

 common, and 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 structural character is what is termed lenticular or 

 wedge-shaped, in which the upper and under surfaces lack paral- 

 lelism, and the beds wedge out. It makes the quarrying more 

 difficult, and produces more waste material. 



These 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 hardness depends upon the quartz, and the strength 

 of the cement holding the grains or fragments together. With- 

 out the cement, or in the loosely aggregated stone, the grains 

 are readily torn apart, and the mass falls with a blow, — a heap 

 of sand. Generally the more silicious the stone and the cement, 

 the greater the degree of hardness and strength. The size, 

 color and arrangement of the component grains are the 

 elements which affect the appearance and give beauty to 

 the sandstone. The durability is connected intimately with the 

 physical constitution and the chemical composition. As a rule 

 calcareous and clayey cementing materials are not as enduring 

 as the silicious and ferruginous. 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 minerals 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 

 destruction. 



