POPULAR GEOLOGY. INTRODUCTORY CHAPTER. THE MATERIAL ORIGIN OF THE GLOBE. 1. The name Geology is derived from two Greek words, ge, signifying the earth; and logos, a discourse; and which, together, may be translated as reasoning about the structure of the earth, or simply the science of the earth. 2. The objects of Geology are to produce a true history of the origin and structure of the globe; of the changes which it has undergone; of the various tribes of plants and animals which have at different periods occupied its surface; and, lastly, to reason from the known state of things in the past to the probable state of things in the future. 3. Cosmogony, necessarily a part of Geology.-As to the origin of the earth, many Geologists appear to think that, as the utmost efforts of man's mind can only enable him to speculate upon this vast mystery, it should be altogether dismissed from the region of Geological science. Hatton says, Geology is in nowise concerned with questions as to the origin of things; and Lyell, that Geology differs as widely from Cosmogony, as speculations concerning the mode of the first creation of man differ from history. Now, these gentlemen would be the last to say, that between the material mode of originating the world, and its existing structure, there is an impassable barrier; or, in other words, that there is no cause in the first bearing a distinct relation to effects in the second. They both believe just the contrary. Geology, therefore, has, we think, rightly concerned itself with the origin of the globe, and must continue to do so. But let us not be mistaken. Crude speculations on such a theme are worse than useless; they are, so to speak, irreverent. Men should come to such a subject with something of the spirit that imbued those who were privileged to enter the Holy of Holies in the ancient Jewish temples. The high-priests of knowledge alone should walk here. 4. Laplace's Theory. The philosopher whose speculations on the origin of the world have stood without injury the test of time, and an unceasing comparison with all the known phenomena of the solar system, is the French astronomer and mathematician, Laplace. The following theory is founded upon his views. Vast extensions of luminous and heated matter exist in space. In one of these a nucleus is established and becomes a centre of aggregation to the neighbouring particles. As they flow on in varying directions, opposing currents are formed, which meet, and cause a rotatory motion *The scientific word, especially expressive of speculations on the origin and creation of the world. to take place; just as we may see, in the waters of a running stream, little bubbles appear, go round and round, strike against each other, then mingle perhaps into one, and still continue to rotate. As the nucleus-or Sun, for it is that of which we speak--increases in size, so it increases in rapidity; and should condensation take place through the loss of heat, arising from a difference of temperature between the heated nebulous matter and the colder surrounding space, that would also accelerate speed. At last the centrifugal, or flying-off force, overcomes the agglomerating or centripetal force, and mass after mass is thrown off in the form of rings or zones. If these happen to be of uniform constitution, they preserve their shape, as in the instance of the rings of Saturn, thrown off originally in the same way from the planet, as the planet itself was thrown off from the primary body, the Sun. If the zones are not thus uniform in their constitution, they break up into one or more masses, having the same degree of speed and the same orbital line of progress as the parent zone possessed before its separation from the main body, and rotating in consequence of the excess of speed existing in the outer as compared with the inner portion of the zone. Thus, it is presumed, our world and the other planetary bodies was formed from the sun; and thus, by a repetition of the same process, were the moon and other planetary satellites formed from the planets. 5. Original Dimensions of the Sun.-If this view becorrect, the original dimensions of the Sun in its undivided state were identical with the dimensions of our entire solar system; and the subsequent history of its condensation is strikingly told by the several orbits of the planets; each of these marking the Sun's dimensions at the time the planet was dismissed into space, to lead a comparatively independent existence. 6. Original Dimensions of the Earth.—Our globe, again, must have extended to the line now traced by its satellite, the moon; must have been then 482,000 miles in diameter, instead of nearly 8,000 miles, as at present; and must have taken twenty-nine days and a half to rotate on its own axis, instead of twenty-four hours. 7. Common Direction of the Planetary Bodies, evidence of a common origin. Of course, it is indispensable to such a theory, that the planets and their satellites should all show their original unitary movement in their present individual movements. And this they do. The planets have one common direction round the Sun; the satellites move in the same direction, whilst also encircling their respective planets: and both planet and satellites, while revolving each on its own axis, make that revolution also in the same general direction, viz., from west to east, which is the Sun's own movement round its axis. 8. The Origin of the Planetary Bodies also illustrated in their respective densities.-The planets and satellites should also, to be in accordance with this theory, possess varying degrees of density. The heavier portions of the parent body must have been the most central; the lighter, those nearest the extremity, and therefore the first to be thrown off. This also is essentially the truth. The planets nearest the Sun are the most dense, the farthest from it the least so; the exceptions being only such as may be ascribed to some of the lesser influences that may have modified the general law. The order of the chief planets, as regards their different degrees of proximity to the Sun, is-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus: now, Mercury, the planet nearest the Sun, is almost three times as dense as the Earth; whilst the earth itself is nearly four times as dense as Uranus, the body farthest from the Sun of those we have named. 9. Chemical proof of Laplace's Theory, down from the Earth's Elements.— Chemical analysis of the component parts of the earth's body leads us, by a more exact and trustworthy route, to corresponding conclusions. All known substances, however varying in apparent origin, structure, qualities, and uses-from the humblest pebble beneath our feet, up to the highest human organization—may be resolved into about fifty-five elementary substances; and which substances, it is presumed, are not themselves compound; that is to say, are not capable of further division. Forty. of these are metallic bodies, twelve non-metallic, and three intermediate between the two. Turning from the consideration of the foregoing theory of the earth's material origin, to these elements of the earth itself, one naturally asks-Can these bodies have ever been in the state of heated vapour, or "fire-mist," as it has been called? The answer is easy and satisfactory. Four of the non-metallic bodies exist permanently gaseous: these are oxygen, hydrogen, nitrogen, and chlorine. One-half the solid matter of the globe is estimated to consist of oxygen. It forms a fifth of the atmosphere; eight-ninths of water; and a large proportion of every kind of rock. When freed from its connection with solid bodies, it expands to two thousand times its former bulk. What happens to freed oxygen would occur with chlorine and other substances similarly disengaged. Look at water, also-forming ice at a temperature lower than 32°-becoming steam at 212°. All these facts point to the conclusion, that every one of the substances of the globe might be converted, under certain circumstances, into gas; and of course, therefore, tend greatly to confirm the hypothesis, that from gas they all came. 10. Heat the Cause of Original Expansion.-Condensation and expansion are, thus, the two opposite processes which the world exhibits, when its present is compared with its original condition. Cold is connected with the one, heat with the other. Was heat, then, the influence that originally kept in a state of vapour all the varying substances that comprise ourselves, and everything around us that we can touch or see? Most probably it was. On descending into the bowels of the earth, as by means of mines and other deep excavations, we find that as soon as we have passed below the regions affected by the sun and other external influences, there is a constant increase of heat as we descend lower and lower. Miners, as is well known, are often obliged to work absolutely naked. It has been calculated that, for every fifteen or sixteen yards of descent, the heat increases one degree of Fahrenheit. Volcanos, and hot springs of water, seem also to suggest subterranean heat as their origin. 11. Density of the Earth only to be explained by the hypothesis of Central Heat. The known degree of density of the earth is such as can only be explained on the hypothesis of some interior force lessening the effects of the concentration of such a mass of solid matter. The surface rocks are only two and a half times as heavy as water; and although the average density of the globe is more than five times as heavy as water, how small is that proportion still to what must have been the weight of the whole earth were there no antagonistic influences! Water at the depth of 362 miles below the surface acquires by compression the density of quicksilver; whilst marble at |