AQUEOUS ROCKS. 
27 
The most natural and convenient mode of classifying the 
various rocks which compose the earth’s crust, is to refer, in 
the first place, to their origin, and in the second to their rel¬ 
ative age. I shall therefore begin by endeavoring briefly to 
explain to the student how all rocks may be divided into 
four great classes by reference to their difierent origin, or, in 
other words, by reference to the difierent circumstances and 
causes by which they have been produced. 
The first two divisions, which will at once be understood 
as natural, are the aqueous and volcanic, or the products of 
watery and those of igneous action at or near the surface. 
Aqueous Eocks. —The aqueous rocks, sometimes called the 
sedimentary, or fossiliferous, cover a larger part of the earth’s 
surface than any others. They consist chiefiy of mechanical 
deposits (pebbles, sand, and mud), but are partly of chemical 
and some of them of organic origin, especially the limestones. 
These rocks are stratified^ or divided into distinct layers, or 
strata. The term stratum means simply a bed, or any thing 
spread out or strewed over a given surface ; and we infer that 
these strata have been generally spread out by the action of 
water, from what we daily see taking place near the mouths 
of rivers, or on the land during temporary inundations. For, 
whenever a running stream ebarged with mud or sand, has 
its velocity checked, as when it enters a lake or sea, or over¬ 
flows a plainj the sediment, previously held in suspension by 
the motion of the water, sinks, by its own gravity to the 
bottom. In this manner layers of mud and sand are thrown 
down one upon another. 
If we drain a lake which has been fed by a small stream, 
we frequently find at the bottom a series of deposits, dis¬ 
posed with considerable regularity, one above the other; 
the uppermost, perhaps, may be a stratum of i3eat, next be¬ 
low a more dense and solid variety of the same material; 
still lower a bed of shell-marl, alternating with peat or sand, 
and then other beds of marl, divided by layers of clay. 
Now, if a second pit be sunk through the same continuous 
lacustrine formation at some distance from the first, nearly 
the same series of beds is commonly met with, yet with 
slight variations; some, for example, of the layers of sand, 
clay, or marl, may be wanting, one or more of them having 
thinned out and given place to others, or sometimes one of 
the masses first examined is observed to increase in thick¬ 
ness to the exclusion of other beds. 
The term '‘formation^'" which I have used in the above 
explanation, expresses in geology any assemblage of rocks 
which have some character in common, whether of origin, 
