THE SEDIMENTARY ROCKS. The three kinds of layered rocks whose
acquaintance we have made--sandstone, limestone, and shale--are
the leading types of the great group of stratified, or
sedimentary, rocks. This group includes all rocks made of
sediments, their materials having settled either in water upon the
bottoms of rivers, lakes, or seas, or on dry land, as in the case
of deposits made by the wind and by glaciers. Sedimentary rocks
are divided into the fragmental rocks--which are made of
fragments, either coarse or fine--and the far less common rocks
which are constituted of chemical precipitates.

The sedimentary rocks are divided according to their composition
into the following classes:

1. The arenaceous, or quartz rocks, including beds of loose sand
and gravel, sandstone, quartzite, and conglomerate (a rock made of
cemented rounded gravel or pebbles).

2. The calcareous, or lime rocks, including limestone and a soft
white rock formed of calcareous powder known as chalk.

3. The argillaceous, or clay rocks, including muds, clays, and
shales. These three classes pass by mixture into one another. Thus
there are limy and clayey sandstones, sandy and clayey limestones,
and sandy and limy shales.

GRANITE. This familiar rock may be studied as an example of the
second great group of rocks,--the unstratified, or igneous rocks.
These are not made of cemented sedimentary grains, but of
interlocking crystals which have crystallized from a molten mass.