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.  Examining a piece of granite, the most conspicuous crystals which meet the eye are those of feldspar.  They are commonly pink, white, or yellow, and break along smooth cleavage planes which reflect the light like tiny panes of glass.  Mica may be recognized by its glittering plates, which split into thin elastic scales.  A third mineral, harder than steel, breaking along irregular surfaces like broken glass, we identify as quartz.

How granite alters under the action of the weather may be seen in outcrops where it forms the bed rock, or country rock, underlying the loose formations of the surface, and in many parts of the northern states where granite bowlders and pebbles more or less decayed may be found in a surface sheet of stony clay called the drift.  Of the different minerals composing granite, quartz alone remains unaltered.  Mica weathers to detached flakes which have lost their elasticity.  The feldspar crystals have lost their luster and hardness, and even have decayed to clay.  Where long-weathered granite forms the country rock, it often may be cut with spade or trowel for several feet from the surface, so rotten is the feldspar, and here the rock is seen to break down to a clayey soil containing grains of quartz and flakes of mica.

These are a few simple illustrations of the surface changes which some of the common kinds of rocks undergo.  The agencies by which these changes are brought about we will now take up under two divisions,—­chemical agencies producing rock decay and mechanical agencies producing rock disintegration.