A VERY little attention to the phenomena of the mineral kingdom, is sufficient to convince us, that the condition of the earth's surface has not been the same at all times that it is at the present moment. When we observe the impressions of plants in the heart of the hardest rocks; when we discover trees converted into flint, and entire beds of limestone or of marble composed of shells and corals; we see the same individual in two states, the most widely different from one another; and, in the latter instance, have a clear proof, that the present land was once deep immersed under the waters of the ocean. If to this we add, that many masses of rock, the most solid and compact, consist of no other materials but sand and gravel; that, on the other hand, loose gravel, such as is formed only in beds of rivers, or on the sea shore, now abounds in places remote from both: if we reflect, at the same time, on the irregular and broken figure of our continents, and the identity of the mineral strata on opposite sides of the same valley, or the same inlet of the sea; we shall see abundant reason to conclude, that the earth has been the theatre of many great revolutions, and that nothing on its surface has been exempted from their effects. To trace the series of these revolutions, to explain their causes, and thus to connect together all the indications of change that are found in the mineral kingdom, is the proper object of a THEORY OF THE EARTH. But, though the attention of men may be turned to the theory of the earth by a very superficial acquaintance with the phenomena of geology, the formation of such a theory requires an accurate and extensive examination of those phenomena, and is inconsistent with any but a very advanced state of the physical sciences. There is, perhaps, in those sciences, no research more arduous than this; none certainly where the subject is so complex; where the appearances are so extremely diversified, or so widely scattered, and where the causes that have operated are so remote from the sphere of ordinary observation. Hence the attempt! to form a theory of the earth are of very modern origin, and as, from the simplicity of its subject astronomy is the eldest, so, on account of the complexness of its subject, geology is the youngest of the sciences.