GEOLOGICAL POSITION AND GEOGRAPHICAL DISTRIBUTION 21 5 



sandstones are marked by the presence of carbonate of lime. 

 When it exceeds the quartz in amount the sandstone 

 becomes a siliceous limestone. In the argriHaceous varieties 

 the binding material is a clay or an impure kaolin. 



The cementing material determines in most cases the 

 color. The various shades of red and yellow depend upon 

 the iron 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 oxide, or carbonate.'^ By an irregular associa- 

 tion 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 

 thickness, 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 directions, and is known as freestone. 

 When fine-grained 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 parallelism, and the beds 

 wedge out. It makes the quarrying more difficult, and pro- 

 duces more waste material. 



These variations in the nature of the component grains, 

 and binding material, in their arrangement and in the forms 

 of bedding, produce a great variety of stone, and the grada- 

 tions from one to another are slight. The hardness, 

 strength, beauty and durability are determined by these 

 varying elements of constitution. The hardness depends 

 upon the quartz and the strength of the cement holding the 

 grains or fragments together. Without the cement, or in 



*W. G. Mann, Quart. Jour. Geol. Soc, Vol. xxiv, p. 355. 

 4 



