38 GEOLOGY 



The diatom, radiolarian, and other oozes (Fig. 15) of the deep sea 

 are the great examples. Sometimes layers of infusorial earth 

 (tripolite) are made up of the secretions of diatoms and other 

 aquatic organisms, accumulated in rather shallow waters. The 

 most familiar examples of indurated rock formed in this way are 

 certain flints and cherts that occur in limestone and chalk, chiefly 

 as nodules, but sometimes in distinct beds. 



Precipitation from solution. Sedimentary rock is formed in 

 still other ways, as by direct precipitation from water which is 

 saturated. Thus limestone and dolomite might be formed by direct 

 precipitation from water if it became saturated with CaCO 3 and 

 MgCO 3 , and some limestone has been formed in this way. Rock- 

 salt has been deposited in thick beds at various times and places, 

 as it is being desposited now about Salt Lake in Utah. The 

 sodium of the salt (composition NaCl) doubtless came from decay- 

 ing rock, for many igneous rocks contain a little sodium in some 

 complex combination. In the decay of the rock, the sodium is 

 taken out of its complex combination, and made into some soluble 

 compound, and then taken to the sea or to a lake. Its union with 

 chlorine makes common salt. Gypsum (CaSO 4 ) is another form 

 of rock deposited in a similar way. Iron ore often occurs in such 

 large bodies that it must be called rock, and some of these bodies 

 of ore are formed by precipitation from solution. 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 by the accumulation of vegetable 

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

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



The following table gives the principal classes of sedimentary 

 rocks: 



Mechanically formed 

 Clastic 



Chemically formed 

 Non-clastic 



Conglomeratic rocks, gravel, conglomerate, breccia, 



etc. 



Arenaceous rocks, sand, sandstone, some arkose, etc. 

 Argillaceous rocks, clay, shale, etc., somelime stones. 

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

 Chloride rocks, especially rock-salt. 

 Sulphate rocks, especially gypsum. 

 Some siliceous rocks, some cherts, etc. 



