We now pass to the third division of Aqueous Rocks, those, namely, which are believed to have come into existence chiefly through the agency of animal and vegetable life, and are therefore called Organic. The study of these rocks has been prosecuted with no inconsiderable ardor during the last thirty years; and the facts which have been brought to light are certainly amongst the most curious and interesting in the whole range of physical science. Indeed we are convinced that a simple narrative of the researches which have recently been made upon this subject, and the discoveries to which these researches have led, would be no less attractive, and scarcely less wonderful, than a fairy tale. But it is not for us to wander at large over this vast and tempting field of inquiry. We must be content with one or two examples, which may help to illustrate the process of inductive reasoning upon which the general principles of geological science are founded. It is argued, then, that the present operations of Nature afford the best key for the interpretation of her works in bygone times. We observe various beds of rocks now in course of formation on the surface of the Earth; and within the Crust of the Earth we discover corresponding strata of the self-same rock already complete, and laid by, as it were, in Nature’s storehouse. Side by side, therefore, we may study and compare the finished work and the work that is yet in progress; and if, on a close examination, they are found to agree in all essential characters, we have doubtless a strong presumption, that the same causes which are now producing the one, must in former times have produced the other. This line of argument we have already considered in reference to those two classes of Aqueous Rocks, which are said to be respectively of Mechanical and of Chemical origin. We now proceed to show that it is no less applicable to those which are called Organic. And although we may not hope to unfold all the secret wonders of Nature’s laboratory, that have come to light in recent times, yet we may afford a passing glimpse at her operations, which can scarcely fail to be interesting and instructive.