206 THE SEDIMENTARY ROCKS 



and the one which best resists chemical change. Small quantities 

 of other minerals, such as magnetite, mica, felspar, garnet, etc., are 

 generally present. 



Sand is made up of fine grains of quartz, not compacted to- 

 gether, but forming a loose, incoherent mass. River sands and 

 those formed by the atmospheric disintegration of rocks commonly 

 have angular grains, due to the splitting up of the quartz fragments 

 along preexisting flaws. Beach sands are more apt to be rounded, 

 due to the constant wash of the surf. 



Sandstone is a rock of varying degrees of hardness, the grains 

 of sand being held together by a cement. The most important 

 cementing substances are carbonate of lime, the oxides of iron, 

 and silica. The sandstones with calcareous cement usually yield 

 quickly to the action of the weather, because of the solubility of 

 the cement. Those with ferruginous cement are much more 

 durable and more highly coloured, being of various shades of red, 

 yellow, and brown. Most durable of all are the siliceous cements. 



Novaculite (or oilstone) is an exceedingly dense and fine-grained 

 sandstone, the particles of which are as fine as those of clay. Its 

 smoothness and hardness fit it admirably for sharpening fine tools. 

 Extensive deposits of this rock occur in Arkansas. 



Varieties of sandstone are produced by the conspicuous admixt- 

 ure of other minerals ; thus, micaceous sandstone has abundant 

 flakes and spangles of mica deposited along the planes of strati- 

 fication. Argillaceous sandstone is composed of a more finely 

 grained sand than the more typical sandstones, contains consider- 

 able quantities of clay, and is, in general, more thinly bedded. 

 The flagstones, so largely used for pavement, are examples of such 

 a rock, and split readily into slabs of almost any desired size. 



Arkose or Felspathic Sandstone is a rock composed largely of 

 cemented grains and fragments of felspar, which have been me- 

 chanically broken up by the action of water, but not chemically 

 disintegrated. More or less sand is often mingled with the felspar 

 grains. 



Breccia is a rock made of large angular fragments cemented 

 together. The fragments may be of any kind of material. 



