his talents as a naturalist. And M. Agassiz has shown himself worthy of his great predecessor in geological natural history, not only by his acuteness and activity, but by the comprehensive character of his zoological philosophy, and by the courage with which he has addressed himself to the vast labours which lie before him. In his Report on the Fossil Fish discovered in England, published in 1835, he briefly sketches some of the large questions which his researches have suggested; and then adds,7 'Such is the meagre outline of a history of the highest interest, full of curious episodes, but most difficult to relate. To unfold the details which it contains will be the business of my life.'

[2nd Ed.] [In proceeding downwards through the series of formations into which geologists have distributed the rocks of the earth, one class of organic forms after another is found to disappear. In the Tertiary Period we find all the classes of the present world: Mammals, Birds, Reptiles, Fishes, Crustaceans, Mollusks, Zoophytes. In the Secondary Period, from the Chalk down to the New Red Sandstone, Mammals are not found, with the minute exception of the marsupial amphitherium and phascolotherium in the Stonesfield slate. In the Carboniferous and Devonian period we have no large Reptiles, with, again, a minute amount of exception. In the lower part of the Silurian rocks, Fishes vanish, and we have no animal forms but Mollusks, Crustaceans and Zoophytes.

The Carboniferous, Devonian and Silurian formations, thus containing the oldest forms of life, have been termed paleozoic. The boundaries of the lifebearing series have not yet been determined; but the series has in which vertebrated animals do not appear been provisionally termed protozoic, and the lower Silurian rocks may probably be looked upon as its upper members. Below this, geologists place a hypozoic or azoic series of rocks.

Geologists differ as to the question whether these changes in the inhabitants of the globe were made

7 Brit. Assoc. Report, p. 72.

by determinate steps or by insensible gradations. M. Agassiz has been led to the conviction that the organized population of the globe was renewed in the interval of each principal member of its formations.8 Mr. Lyell, on the other hand, conceives that the change in the collection of organized beings was gradual, and has proposed on this subject an hypothesis which I shall hereafter consider.]

Sect. 2.-Transition to Geological Dynamics.

WHILE we have been giving this account of the objects with which Descriptive Geology is occupied, it must have been felt how difficult it is, in contemplating such facts, to confine ourselves to description and classification. Conjectures and reasonings respecting the causes of the phenomena force themselves upon us at every step; and even influence our classification and nomenclature. Our Descriptive Geology impels us to endeavour to construct a Physical Geology. This close connexion of the two branches of the subject by no means invalidates the necessity of distinguishing them: as in Botany, although the formation of a Natural System necessarily brings us to physiological relations, we still distinguish Systematic from Physiological Botany.

Supposing, however, our Descriptive Geology to be completed, as far as can be done without considering closely the causes by which the strata have been produced, we have now to enter upon the other province of the science, which treats of those causes, and of which we have already spoken, as Physical Geology. But before we can treat this department of speculation in a manner suitable to the conditions of science, and to the analogy of other parts of our knowledge, a certain intermediate and preparatory science must be formed, of which we shall now consider the origin and progress.

GEOLOGICAL DYNAMICS.

CHAPTER V.

INORGANIC GEOLOGICAL DYNAMICS.

Sect. 1.-Necessity and Object of a Science of Geological Dynamics.

WHEN the structure and arrangement which men

observed in the materials of the earth instigated them to speculate concerning the past changes and revolutions by which such results had been produced, they at first supposed themselves sufficiently able to judge what would be the effects of any of the obvious agents of change, as water or volcanic fire. It did not at once occur to them to suspect, that their common and extemporaneous judgment on such points was far from sufficient for sound knowledge;-they did not foresee that they must create a special science, whose object should be to estimate the general laws and effects of assumed causes, before they could pronounce whether such causes had actually produced the particular facts which their survey of the earth had disclosed to them.

Yet the analogy of the progress of knowledge on other subjects points out very clearly the necessity of such a science. When phenomenal astronomy had arrived at a high point of completeness, by the labours of ages, and especially by the discovery of Kepler's laws, astronomers were vehemently desirous of knowing the causes of these motions; and sanguine men, such as Kepler, readily conjectured that the motions were the effects of certain virtues and influences, by which the heavenly bodies acted upon each other. But it did not at first occur to him and his fellow-speculators, that they had not ascertained what motions the

influences of one body upon another could produce; and that, therefore, they were not prepared to judge whether such causes as they spoke of, did really regulate the motions of the planets. Yet such was found to be the necessary course of sound inference. Men needed a science of motion, in order to arrive at a science of the heavenly motions: they could not advance in the study of the Mechanics of the heavens, till they had learned the Mechanics of terrestrial bodies. And thus they were, in such speculations, at a stand for nearly a century, from the time of Kepler to the time of Newton, while the science of Mechanics was formed by Galileo and his successors. Till that task was executed, all the attempts to assign the causes of cosmical phenomena were fanciful guesses and vague assertions; after that was done, they became demonstrations. The science of Dynamics enabled philosophers to pass securely and completely from Phenomenal Astronomy to Physical Astronomy.

In like manner, in order that we may advance from Phenomenal Geology to Physical Geology, we need a science of Geological Dynamics; that is, a science which shall investigate and determine the laws and consequences of the known causes of changes such as those which Geology considers;-and which shall do this, not in an occasional, imperfect, and unconnected manner, but by systematic, complete, and conclusive methods;-shall, in short, be a Science, and not a promiscuous assemblage of desultory essays.

The necessity of such a study, as a distinct branch of geology, is perhaps hardly yet formally recognized, although the researches which belong to it have, of late years, assumed a much more methodical and scientific character than they before possessed. Mr. Lyell's work (Principles of Geology) in particular, has eminently contributed to place Geological Dynamics in its proper prominent position. Of the four books. of his Treatise, the second and third are upon this division of the subject; the second book treating of aqueous and igneous causes of change, and the third, of changes in the organic world.

There is no difficulty in separating this auxiliary geological science from theoretical Geology itself, in which we apply our principles to the explanation of the actual facts of the earth's surface. The former, if perfected, would be a demonstrative science dealing with general cases; the latter is an ætiological view having reference to special facts: the one attempts to determine what always must be under given conditions; the other is satisfied with knowing what is and has been, and why it has been: the first study has a strong resemblance to Mechanics, the other to philosophical Archæology.

Since this portion of science is still so new, it is scarcely possible to give any historical account of its progress, or any complete survey of its shape and component parts. I can only attempt a few notices, which may enable us in some measure to judge to what point this division of our subject is tending.

We may remark, in this as in former cases, that since we have here to consider the formation and progress of a science, we must treat as unimportant preludes to its history, the detached and casual observations of the effects of causes of change which we find in older writers. It is only when we come to systematic collections of information, such as may afford the means of drawing general conclusions; or to rigorous deductions from known laws of nature;-that we can recognize the separate existence of geological dynamics, as a path of scientific research.

The following may perhaps suffice, for the present, as a sketch of the subjects of which this science treats: the aqueous causes of change, or those in which water adds to, takes from, or transfers, the materials of the land:-the igneous causes; volcanoes, and, closely connected with them, earthquakes, and the forces by which they are produced;-the calculations which determine, on physical principles, the effects of assumed mechanical causes acting upon large portions of the crust of the earth;-the effect of the forces, whatever they be, which produce the crystalline texture of rocks, their fissile structure, and the separa