248 PHYSICAL GEOLOGY 



Consolidation of Sediments 



The greater part of the surface of the continents of the world is 

 composed of sedimentary rocks which were originally laid down in 

 the sea. These were at first largely unconsolidated gravels, muds, 

 and calcareous oozes, but are now usually thoroughly consolidated. 

 In general, it may be said that the more recent rocks are not as firm as 

 those of greater age. For example, the rocks 1 of the coastal plains 

 of the United States (p. 574) are sands and clays which are almost 

 or quite as soft as when first laid down ; while in other portions of 

 the continent, where the sedimentary rocks are older, they are usually 

 hard. The consolidation of sedimentary rocks is brought about by 

 (1) cementation, (2) pressure, and (3) heat. 



Cementation. — Loose (incoherent) deposits are consolidated either 

 by direct cementation or by the formation of interlocking, fibrous 

 crystals which hold the grains firmly together. Some recently built 

 sand reefs, such as those which border the coast of Pernambuco, Brazil, 

 are already converted into sandstone by the deposition of calcium car- 

 bonate between the sand grains. At the mouths of rivers the sedi- 

 ments are sometimes consolidated by calcium carbonate, precipitated 

 from the fresh water as it mingles with the sea water, as rapidly 

 as they are laid down. In deposits composed of fragments of shells, 

 calcium carbonate also constitutes the cementing material. In this 

 case, as has been seen (p. 51), the sediment furnishes its own 

 cement, which is first dissolved from the calcareous fragments and 

 later redeposited a short distance beneath the surface of the de- 

 posit, thus forming a more or less compact limestone. In this way 

 the limestone of Bermuda and the " coquina " limestone of the coast 

 of Florida were formed. 



Sediments are also cemented by iron oxide which is derived from 

 the soluble salts of iron carried into seas and lakes. These iron 

 compounds upon oxidation sink to the bottom and firmly cement 

 the sand. 



Sands composed of quartz grains are sometimes cemented by 

 silica and form extremely hard quartzites (p. 344). 



Effect of Pressure. — When subjected to the great pressure of 

 overlying sediments, muds are compacted and thus hardened into 

 shale. The compactness of shale is also sometimes increased by the 



1 The word rock, used technically, does not necessarily imply compactness, but includes 

 loose sands as well as granites. 



