6. The _Eolian_ or _Aërial_ group of rocks embraces all natural accumulations of organic or inorganic materials, which have been formed upon the land. The group is typically represented by _débris_, such as gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of deserts and maritime districts, and by _soil_. All these accumulations owe their origin to atmospheric agencies, as will be more particularly described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the results of the _mechanical_ action of water and the atmosphere, they are fitly arranged under one great class--the MECHANICALLY FORMED ROCKS.
7. CHEMICALLY FORMED ROCKS constitute another well-marked class, of which we may take _rock-salt_ as a typical example. This rock has evidently been deposited in water, but not in the manner of a sedimentary bed. It is not built up of water-worn particles which have been rolled about and accumulated layer upon layer, but has been slowly precipitated during the gradual evaporation of water in which it was previously held in solution. Its formation is therefore a chemical process. Various other rocks come under the same category, as we shall afterwards point out.
8. The ORGANICALLY DERIVED class comprises a number of the most important and useful rock-masses. _Chalk_ may be selected as a typical example. Even a slight examination shews that this rock differs widely from any of those mentioned above. Conglomerate, sandstone, shale, &c. are built up of pebbles, particles, grains, &c. of various inorganic materials. But chalk, when looked at under the microscope, betrays an organic origin. It consists, chiefly, of the hard calcareous parts of animal organisms, and is more or less abundantly stocked with the remains of corals, shells, crustaceans, &c. in every degree of preservation; indeed, so abundant are these relics, that they go to form a great proportion of the rock. _Coal_ is another familiar example of an organically derived rock, since it consists entirely of vegetable remains.
9. The METAMORPHIC class, as the name implies, embraces all those rocks which have undergone some decided change since the time of their formation. This change generally consists in a re-arrangement of their constituent elements, and has frequently resulted in giving a crystalline texture to the rocks affected. Hence certain sedimentary deposits like sandstone and shale have been changed from granular into crystalline rocks, and the like has happened to beds of limestone and chalk. _Mica-schist, gneiss_, and _saccharoid marble_ are typical of this class.