THE COMPOSITION OF THE EARTH 37 



of gravel, sand, and mud are formed in the same place. This 

 division into layers, or stratification, as it is called, is the most 

 universal and important characteristic of water-laid beds, 

 though not confined to them. 



Countless numbers of minute organisms live in the clear and 

 relatively quiet waters beyond the reach of abundant land- 

 derived sediment. Many of these organisms take calcium 

 carbonate or silica from solution in sea water, and build it into 

 their shells and other hard parts. When they die, these 

 shells, etc., sink, and over millions of square miles of the ocean 

 floor form a deposit, called ooze (Fig. 277, p. 259). Near the 

 shores, these organic deposits are usually less important than 

 the gravels, sands, and muds brought down from the land. 



The consolidation of sediments. Material in solution in 

 sea water is sometimes deposited among the particles of the 

 sediment, binding them together to form firm, solid rocks. 

 Furthermore, the bottom sediment is under the weight of the 

 overlying material deposited later, and this may become effec- 

 tive in pressing the particles closer together, though it prob- 

 ably does not aid greatly in making the mass coherent. 



Sea-laid sediments may be exposed by an elevation of the 

 ocean bottom, or by a lowering of the sea surface, and ground 

 waters containing minerals in solution may subsequently 

 deposit material in their pores, further cementing the rocks. 

 Rock cementation is often a very slow process, and coastal 

 plains that have emerged from the sea recently (as geology 

 measures time) are apt to be underlain by beds of loose 

 material rather than of solid rock. This is generally true in 

 the Atlantic Coastal Plain of the United States (Figs. 43 and 

 44, pp. 62, 63). 



Certain sediments (particularly lime carbonate oozes) are 

 consolidated not only by cementation, but also, and in some 

 cases chiefly, by the formation of minute interlocking crystals. 



Chief kinds of sedimentary rocks. Sedimentary rocks are 

 formed from loose sediments by (1) cementation, (2) crystal- 

 lization (in some cases), and (3) pressure (to slight extent), 



