154 MINERALS AND GEOLOGY 



existing rock-masses, by the wearing away or destruction of these ; 

 ^nd secondly, that they have all been formed or deposited under water. 



In pursuance of this inquiry, consequently, we have to consider, 

 first, the origin or derivation of the sediments of which these rocks 

 -are made up ; and, secondly, the processes by which the consolidation 

 of the sediments into rock, properly so-called, was affected. 



The sediments of which these rocks originally consisted, were de- 

 rived from previously-existing rocks, by decomposing atmospheric 

 agencies rain, frost, and so forth ; by the action of streams and rivers 

 on their beds ; and by the destructive action of the waves and breakers 

 of the sea. 



Action of the Atmosphere. All rocks, even the most solid, are con- 

 stantly undergoing decomposition and decay. The exposed face of a 

 rock of any kind, for example, soon changes colour, and becomes in 

 general more porous than the other portions of the rock. This effect 

 is technically termed " weathering." Its action gives rise to the pro- 

 duction of soils, and frequently causes the fossils contained in "the rock 

 to stand out in relief, these bodies being in many cases less easily 

 destructible than the mass of rock itself. Every shower of rain that 

 falls, takes part in this decomposing or disintegrating action, and 

 carries off something, in s lution or suspension, to lower levels id 

 est, into streams, lakes, and seas. Frost, and, in certain localities, 

 carbonic acid and other gases issuing through crevices in the rocks, 

 assist this destructive process. Rain, acting on loosely coherent mat- 

 teis, is known in many districts to have excavated channels of con- 

 siderable extent. These may become in coarse of time more or less 

 permanent water courses, and the work of excavation be thus con- 

 tinously carried on. 



Action of Streams and Rivers. The action of streams and rivers, 

 in wearing their channels, is both chemical and mechanical. Calca- 

 reous river-beds are wasted bit by bit by the dissolving power of the 

 water, especially during the autumnal season, \vhen dead leaves and 

 other decaying vegetable matters yield the water a large supply of 

 carbonic acid. On the other hand, a mechanical waste is also very 

 .generally taking place to a greater or less extent ; and thus numerous 

 Drivers are continually cutting back their beds, and forming ravines. 

 The Falls of the Niagara River have in this manner gradually receded 

 from the face of the escarpment near Queenston to their present site ; 



