[In the preceding section the Author has noticed the _superficies_ of the earth principally; as its inequalities because of seas, lakes, rivers, mountains, vallies, &c. The _rocky_, and _earthy_ masses and strata, which cover the nucleus of our globe, are scarcely mentioned at all. Whether the _central_ parts of the earth be solid, soft, or hollow, and filled with gaseous matter, is not the subject of enquiry here: but the _composition_ and _arrangement_ of the _solid crust_ of the planet come under consideration. As it regards the composition of the crust of the earth considered principally, it consists of _metallic oxides_. The bases of the different earths are well known to be _metals_. The metal called _Silicon_, is the base of silex or flint--_Aluminum_ is the metallic base of pure clay--_Calcium_, of lime--_Magnesium_, of magnesia--_Potasium_, of potash, &c. Iron, also, enters largely into the composition; and soda, whose metallic base is _sodium_, forms a considerable portion. These bases, at their creation, existed in an _uncombined_ state, as did all the elementary substances. When they entered into combination with _oxygen_ they became _earths_, which are simple metallic oxides, which readily combine with the _acids_, in which combination they are generally seen, though not always, at the earth’s surface; as carbonate of lime, or common limestone; the composition of which is _calcium_, _oxygen_, and _carbonic acid_. Rocks of the _silicious_ family are not considered _earthy salts_, though, occasionally, they may contain a small per cent. of acid. They are called _earthy compounds_. _Granite_ is an instance; composed of _feldspar_, _quartz_, and _mica_. Gneiss, and mica slate are of similar composition, though in different proportions, and under different arrangements. It will readily occur to the reader that there are some other earths, and other substances also, as the acids, and gases, which enter into the composition of the earth’s crust, though in small proportions, and, therefore, are not considered _principal_ ingredients, and hence not noticed in this general sketch. The rocky, or stony substances, composed of the above elements, under the influence of chemical affinities, and other principles, are found in _crystalline_, _stratified_, _amorphous_, and _aggregate masses_. The _position_, _structure_, and _contents_ of these masses will develope the _natural history of the solid crust of our Earth_. In order to facilitate this development, the rocks have been divided, according to their age into, 1. _Primitive Rocks._ These were deposited _first_, as is evident from their position, being the lowest of all the rocks. Their name indicates their relative age.