had this clue been followed, the leading principles of geology would have been ante-dated many centuries.

Throughout the middle ages, the world, plunged in the sea of theological controversy, found no leisure for the calm contemplation of science; and the best minds were drawn into the vortex which swallowed up all independent thought. Theology demanded concurrence with its own view of the creation of the earth and its inhabitants, under severe penalties; burked inquiry whenever this was possible; and explained away indisputable facts by fanciful theories, or met them by dogmatic assertion. Science owes much to the Reformation. Thenceforward it was not a question of choice between the stake and assertion of the truth of some law of nature; but the human intellect found itself released from the fetters which had so long defeated its utmost struggles for freedom. From the chaotic mass accumulated by astrologers, Copernicus, Kepler, and Newton evolved order in the movements of the celestial bodies; and later, Laplace enunciated the theory of the origin of the universe which we accept, with unimportant modifications, at this day.

The mathematicians had demonstrated the unity of the dynamical laws which governed the movements of all members of the system, and had made new discoveries solely on the faith of the universal application of those laws; and Laplace suspected that bodies, which so rigidly conformed to certain laws of motion, might possess a common composition and origin. Hence the celebrated "nebular hypothesis," which the spectroscope has sustained so remarkably in the hands of Huggins,

Frankland, Lockyer, Secchi, and others. Laplace supposed that all the members of our solar system were the result of the condensation of a greatly attenuated nebulous mass, possibly extending to the utmost limits of our system, eventually splitting up into portions of various magnitudes, and circulating, in proportion to their mass, at different distances from the largest portion-the sun-in virtue of the well-established laws of gravitation. If, then, all the bodies had this common origin, similarity, if not identity, in their composition might be looked for, and has actually been found to exist by modern researches in physics and chemistry. Being no longer strangers to us and we to them [but closely associated in origin and material composition with our earth], the sun, planets, stars, comets, and nebulæ began to assume new interest, and to stimulate new methods of inquiry directed to their physical history, present and past condition, and even possible future. Associated so closely as they had been in their birth, it could not be but they had similar characters in their maturity, and might also undergo dissolution from identical causes. Improvements in the telescope made the interesting revelation that Jupiter, and probably the other most distant planets, were worlds in process of formation; that Mars was in all essential respects in a stage of development comparable to that of the earth; and that the moon had progressed far towards dissolution-conclusions which supported the hypotheses of Laplace. These and like considerations directed stricter attention to the structure of our

own planet-What had been its history since its independent existence ?

Modern engineering works, particularly mining, have supplied the answer to this question; but not before many different opinions had been advanced and proved untenable. Passing over the sixteenth and seventeenth centuries-during which we see signs of a desire to penetrate that which was considered a mystery, rather than any systematic efforts to collect and interpret facts -the eighteenth heralded a new era in the inquiry, and the foundation of geology was laid, loosely perhaps as we now regard it, but on intelligible lines. The names of Werner and Hutton can never be remembered but with gratitude; for the very fierceness of the controversy aroused by their conflicting doctrines showed that nothing could be done without patient observation, and extreme caution in generalization while the known facts were so few and isolated. Just before the beginning of this century, William Smith's "tabular view" gave coherence to an undigested mass of observations gathered at random from various sources, and firmly established the principle of the succession of strata and their identification by means of fossil remains.

The progress of the science since then is matter of contemporaneous history-a science adorned by the splendid names of Humboldt, Murchison, D'Orbigny, Lyell, Sedgwick, Mantell, and Hugh Miller, with others who have passed away; and sustained by a body of workers as eminent in research and as conscientious

in investigation as any branch of human knowledge has ever produced.

What is our first impression of the earth? That it is immoveable and unchangeable. Those hills look indeed "everlasting," and oceans and rivers appear to have for ever remained within the same boundaries. But is this so? We feel no motion of the earth, but we know from astronomy that it is spinning round on its axis at the rate of more than a thousand miles an hour, and rushing through space in its orbit at more than sixty thousand miles an hour. We see, then, how very far it is from being immoveable. It is equally far from being unchangeable. The mountains which we look upon towering far above the clouds have not always existed; the oceans have not had the same places. Where land now is, seas and rivers have flowed; and again, where seas and rivers are, dry land has been. All our ideas, then, of the stability and unchangeableness of the earth must be put aside, and we must regard it as the scene of continual disturbance from age to age throughout its long history. Geology has made the story of the earth as plain as astronomy has that of the heavens.

This science is devoted to the study of the origin and classification of the materials of our earth, and of the changes they have undergone in all stages of their formation through the operation of physical forces-such as contraction and displacement, the action of fire, water, air, frost, etc. Geologists designate by the generic term "rock" all these materials, whether they be hard, as granite and lava, or soft, as clay and sand. Each of

them, however, has a specific name, generally derived from some leading character in its composition or the manner of its formation.

Taking a limited view of the surface of the globe, it does not at first appear that there is any great variety of rocks, because they are so frequently concealed by vegetation or overlie one another at any particular spot we may be examining. When, however, we take a wider area into consideration, the variety of them at once becomes apparent. At some portion of the earth one or more of the rocks lies at the surface, and we are thus able to classify them and ascertain the character of the whole of the materials which constitute the globe. At the beginning of this century, very little was known on this subject, and there was scarcely any evidence of the immense age of the earth. Gradually, however, facts have been collected which warrant us in believing that many millions of years have elapsed since the formation of certain of these rocks, and that without the least doubt many thousands of years were occupied in the formation of some rocks of no great thickness comparatively. The reasoning on which these and like conclusions are founded is almost entirely inductive; but it approaches the certainty of demonstration as nearly as any line of reasoning can. Geology cannot claim the precision and exactness of astronomy. Astronomy is based on mathematical problems which, when once stated and proved, are true for all time, past, present, and future; and innumerable proofs of the exactness of this science occur every day. The size, weight, distance,