438 A POPULAR EXPOSITION OF THE 
they are often called Aqueous Rocks. They are chiefly of mechanical 
formation, consisting of muddy, sandy, and other sediments, collected 
by the mechanical action of water, and subsequently consolidated by 
processes described a few pages further on. Various limestones, how- 
ever, and certain other rock matters of this division, are of chemical 
origin, or, in other words, have been deposited from waters in which 
their materials were chemically dissolved. 
These sedimentary or aqueous rocks are characterized by always 
occurring in beds or strata (with the occasional exception of certain 
irregularly-heaped masses of drift materials) ; secondly, by exhibiting 
in many instances, a more or less clearly-marked detrital or sedimen- 
tary structure; and thirdly, by often containing organic remains. 
These latter, comprising shells, bones, leaf-impressions, &c. (see 
Part IV.), are the fossilized parts of animals and plants which lived 
upon the Earth, or in its waters, during the periods in which these 
rocks were under process of formation, as indicated below. 
The sedimentary rocks may be conveniently discussed under the 
following heads: Composition or mineral characters; Modes of 
formation ; Changes to which they have been subjected after deposi- 
tion. 
(1) Composition of Sedimentary Rocks,—Viewed as to their com- 
position, these rocks comprise : 
Sandstones, sands, and gravels—or arenaceous rocks. 
Clays and clay-slates—or argillaceous rocks. 
Limestones and Dolomites—or calcareous rocks. 
Conglomerates and Breccias: rocks of mixed composition (see 
below), 
Trap tafas: stratified deposits formed out of materials derived 
_ from the denudation of trap and greenstone rocks. 
Rock matters of purely organic origin, as coal, &c. 
To these may be added a few other substances of subordinate oc- 
currence, as gypsum and rock-salt. 
Sandstones are nothing more than beds of consolidated sand. 
They are of various colours, but chiefly white, or dull shades of 
yellow, red, brown, or green. The harder and purer kinds, as some ex- 
amples of our ‘“ Potsdam sandstone,”’ are called guartzose sandstones. 
In other kinds, a certain amount of carbonate of lime is present, cement- 
ing together the component grains of sand, and forming calcareous 
sandstones. For special Canadian localities of these and other rocks 
