268 MATERIALS AND THEIR ARRANGEMENT 



most) cases it appears to have taken place while the material of 

 . the limestone was being deposited. 



Siliceous deposits. In the decomposition of igneous rocks, a 

 little of the silica, as well as of the bases, is dissolved and carried 

 away in solution. Certain organisms extract this silica from the 

 water for their tests, shells, etc., just as others extract calcium car- 

 bonate. Siliceous secretions may form siliceous rocks. Diatom and 

 radiolarian oozes of the deep sea are examples. Familiar examples 



Fig. 243. Globigerina ooze, similar to chalk in composition. Magnified 20 

 times. (Murray and Renard.) 



of indurated rock formed in this way are certain flints and cherts 

 that occur in limestone, both as nodules and in distinct beds. 

 Some of these are developed about fossil sponges. 



Precipitation from solution. Some sedimentary rock is formed 

 by direct precipitation from water which is saturated. Thus 

 limestone might be formed by direct precipitation from water if it 

 became saturated with CaCOs, 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 deposited now about Salt Lake in 

 Utah. The sodium of the salt (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 oTit 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 (CaS0 4 ) is another form of 



