SEDIMENTARY ROCKS 269 



rock deposited in a similar way. Iron ore occurs in large bodies, 

 and some of them were formed by precipitation. Salt, gypsum, 

 limestone, and iron ore are peculiar among rocks in that but one 

 mineral enters into their composition when they are pure. 



Coal is a sort of rock formed from accumulations of vegetable 

 matter. Some other sedimentary rocks, as noted above, are formed 

 organically, though they can hardly be said to be organic. 



The principal classes of sedimentary rocks are given below: 



f Conglomeratic rocks, gravel, conglomerate, breccia, etc. 

 Mtvhanically formed J Arenaceous rocks, sand, sandstone, some arkose, etc. 

 Clastic I Argillaceous rocks, clay, shale, etc. 



t A few limestones. 



f Some carbonate rocks, e. g., travertine, siderite. 

 Chemically formed J Chloride rocks, especially rock-salt. 

 Non-cLislii 1 Sulphate rocks, especially gypsum. 



I Some siliceous rocks, some cherts, etc. 



( )r^anirally formed f Calcareous rocks, most limestones. 



A r o-c/(/.v//<: t Siliceous rocks, siliceous oozes, sinter, etc. 



I Carbonaceous, coal, etc. |X 



Distinctive Features of Sedimentary Rocks 



Stratification. Most sedimentary rocks are arranged in more 

 or less distinct layers; that is, they are stratified (Fig. 2). Stratifi- 

 cation consists primarily in the superposition of layers one on an- 

 other. Layers of like constitution or compactness may be sep- 

 arated by films of different material which cause the partings. The 

 bedded arrangement is due to various causes, but primarily to the 

 varying agitation of the waters in which the sediments were laid 

 down. Where depositing waters are agitated vigorously to the 

 bottom, coarse sediment only is deposited. Where waters are 

 quiet at the bottom, fine sediment is the rule. Since the agitation 

 of waters is subject to frequent change, coarser sediment succeeds 

 finer, and vice versa, in the same place. Hence arise beds, layers, 

 and lamina. The terms layer and bed generally are used as syno- 

 nyms, while lamina are thinner divisions of the same sort. The 

 term stratum is sometimes applied to one layer, and sometimes to 

 all the consecutive layers of the same sort of rock. For the latter 

 meaning the term formation is often used. 



The commoner sorts of bedded rock are limestones, shales, 

 sandstones, and conglomerates. In many places the bedding of 

 limestone is caused by films of clayey matter between the layers, 



