These appearances may all be comprehended under two heads, which are now to be mentioned, in order to see the importance of their explanation, or purpose which such an explanation is to serve in a theory of the earth. The first kind of these appearances is that of known bodies which we find composing part of the masses of our land, bodies whose natural history we know, as having existed in another state previous to the composition of this earth where they now are found; these are the relicts or parts of animal and vegetable bodies, and various stony substances broken and worn by attrition, all which had belonged to a former earth. By means of these known objects, we are to learn a great deal of the natural history of this earth; and, it is in tracing that history, from where we first perceive it, to the present state of things, that forms the subject of a geological and mineralogical theory of this earth. But, we are more especially enabled to trace those operations of the earth, by means of the second kind of appearances, which are now to be mentioned. These again are the evident changes which those known bodies have undergone, and which have been induced upon such collected masses of which those bodies constitute a part. These changes are of three sorts; _first_, the solid state, and various degrees of it, in which we now find those masses which had been originally formed by the collection of loose and incoherent materials; _secondly_, the subsequent changes which have evidently happened to those consolidated masses which have been broken and displaced, and which have had other mineral substances introduced into those broken and disordered parts; and, _lastly_, that great change of situation which has happened to this compound mass formed originally at the bottom of the sea, a mass which, after being consolidated in the mineral region, is now situated in the atmosphere above the surface of the sea.