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CHAPTER V. 



THE NATURE, COMPOSITION, AND ORIGIN OF THE COMMON 

 WATER-FORMED ROCKS. 



ALMOST the whole of the land surface of the world consists of 

 rocks which have been accumulated under water. It nowhere shows 

 a trace of such materials as would have resulted from an original 

 igneous fusion ; for such surface rocks would have been uncrystalline 

 and like modern volcanic lavas. Only occasionally are large masses 

 of crystalline rocks seen, and those are only exposed by the water- 

 formed rocks which covered them having been removed by the 

 denuding power of water. Though volcanoes are numerous, the areas 

 of the earth's surface covered with sheets of lava poured out from 

 the earth in a molten state are not very considerable. The whole 

 land surface consists of materials which may be classed as superficial 

 volcanic rocks, deep-seated crystalline rocks, and stratified rocks, which 

 have been spread out under water in layers. The stratified rocks 

 can be accounted for directly or indirectly as products of the wear 

 and tear of the other kinds; and it is almost equally certain that 

 the aqueous rocks may be changed by pressure, and the heat to 

 which pressure gives rise when it is arrested, into the more or less 

 crystalline rocks which are severally named Metamorphic, Plutonic, 

 and Volcanic. We have no means of judging what the earliest- 

 formed rocks were like ; and Professor Huxley, with excellent reasons 

 for the suggestion, has remarked that the oldest rocks now known 

 bear the same relation in point of antiquity to those which must 

 have preceded them, that the newest deposits of the geological series 

 bear to the whole series of strata which have been discovered. 



Kinds of Deposits. The water-formed rocks consist of pebbles, 

 sand, mud, or limestone, which have become hardened by various 

 natural cements into solid beds called strata. The pebbles then 

 become a conglomerate, the sand a sandstone, the mud a clay, and 

 the shells, corals, or foraminifera, or other remains of animals, form lime- 

 stones. These are the chief kinds of water-formed rocks. Pebbles, 

 sand, and clay are worn away by sea, river, or lake water from lands 

 which previously existed, and are spread out parallel to the shore, or 

 at the mouths of rivers. These materials are only held in suspension 

 by the mechanical power of moving water, and hence are often called 

 mechanical deposits ; and since they fall as sediments when the 



