MINERALS AND GEOLOGY OF CANADA. 441 
by the wearing away or destruction of these; and 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 effected. 
The sediments of which these rocks originally consisted, were 
derived from previously-existing rocks, by decomposing atmospheric 
agencies,—rain, frost, and so forth; by the action of streams and 
Tivers 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 
constantly 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 production of soils, and frequently causes the fossils contained in 
the rock to stand out in relief, these being in many cases less easily 
decomposed than the mass of the rock itself. Every shower of rain 
that falls, takes part in this decomposing or disintegrating action, and 
carries off something, in solution or suspension, t6 lower levels—id est, 
into streams, lakes, and seas. Frost, and, in certain districts, carbonic 
acid and other gases issuing through crevices in the rocks, assist this 
destructive process. 
Action of Streams and Rivers.—The action of streams and rivers 
in wearing their channels is both chemical and mechanical. Calcareous 
river-beds are wasted bit by bit by the dissolving power of the water, 
especially during the autumnal season, when 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 rivers are 
continually cutting back their beds, and forming ravines. It is 
thought by many geologists, that the Falls of the Niagara River have 
in this manner gradually receded from the escarpment at Queenston 
to their present site; and there is scarcely a river, or small stream 
indeed, in any part of Canada, that does not exhibit in its banks in- 
dications of erosive action. Where streams wind through the sands 
and gravels of our Drift deposits, as in the neighbourhood of Toronto, 
to cite a single amongst so many instances, examples of this action 
are especially apparent. The River Don, it is said, during a three 
