2. _Transition Rocks._ These rocks are deposited immediately above the primitive, of course subsequently to them. They are called _transition_ rocks, because they were deposited as the earth was _passing_ from an uninhabitable to a habitable state, as is evident from the fact that _they contain the first traces of organized being imbedded in them_. 3. _Secondary Rocks._ These are deposited next in succession to the transition rocks, and mark a _third_ grand geological epoch, by being almost altogether a _mechanical_ deposition, and lie _horizontally_ when _in situ_, and contain an increase of organic remains, both in quantity and variety. 4. _Tertiary Rocks._ These derive their name from their succession to the secondary, and of course mark the _fourth_ geological epoch in the history of the arrangement of the earth’s crust, which completed its redemption from the abyss of waters, and fitted it for the habitation of man. This division of the rocks designates the _order of time_ in which they were successively deposited, as is evident from their position. Considering these rocks _in situ_, they may be reckoned _general formations_, extended all around the globe in concentric circles, as the coats of an onion around its centre, in the order above stated, beginning with the primitive rocks. It is, however, well known that _fractures_ and _dislocations_ prevail to a great extent, the result of violence subsequently to the deposition of these rocks, removing large portions of them _out of place_. But this circumstance need not interrupt the grand _natural_ order of the construction of the earth’s crust. There is also a class of stony substances which follow no general laws, either in regard to _position_, _form_, or _age_. These are volcanic and igneous productions of every kind; as basalt, lava, &c. These shall be mentioned subsequently. In the above remarks we have an _outline_ of the structure of the crust of the earth; but in order to have a more satisfactory development, the principal and distinctive features of the leading rock formations must be stated in order. _Primitive Rocks._ 1. _This class occupies the lowest position as a class_, yet the individual rocks of this class have a general order of position among themselves. Granite is lowest; then Gneiss--Mica Slate--Clay Slate--Primitive Limestone--Porphyry--Sienite--and Greenstone. _These rocks are sometimes observed alternating with each other, and sometimes passing into each other._ But these circumstances do not effect the general order. When the formations are _undisturbed_, in penetrating them we should come to granite last; and it is universally the lowest of all observed rock formations.