Above these rocks we find depositions of sand, and gravel, and clay; accumulations of peat, and other substances now in the progress of deposition, and which are included under the general terms alluvial formations. These constitute the fourth class. The fifth class contains the products of volcanoes, whether the result of fusion or not, such as rocks thrown out without melting, volcanic mud, &c. It is supposed that all the formations as they are termed, or all the different kinds of rocks and strata now found on the earth, will fall under one or another of these classes. But it will be seen hereafter, that this theory is, in many respects, unsatisfactory, and that there are several rocks, such as basalt and greenstone, which certainly are not of aqueous origin, and which do not come within Werner's volcanic class. Werner was appointed professor of mineralogy, at the school of Mines, in Saxony, 1775, and was undoubtedly a man of the highest order of talents. His mind was at once sound, imaginative, and richly stored with miscellaneous knowledge. He had a great aversion to the mechanical labor of writing, and could never be persuaded to pen more than a few brief sketches, and which never contained a connected development of his geological views. Although the natural modesty of his disposition was excessive, approaching even timidity, yet he indulged in the most bold and sweeping generalization, and he inspired all his pupils, some of which became writers of great eminence, with the most implicit faith in his doctrines. Their admiration," says Mr. Lyell," of his genius, and the feelings of gratitude and friendship which they all felt for him, were not undeserved; but the supreme authority which he usurped over his contemporaries, was probably in the event prejudicial to the progress of science." The Plutonic, or Huttonian Theory owes its origin to Dr. Hutton of Edinburgh. It was published in 1788, but has been more recently illustrated and defended in a republication by Professor Playfair, also of Edinburgh. We have already stated that the Plutonians attributed the same phenomena to fire, which the Neptunians did to water. The Plutonians, however, supposed that most stratified rocks were deposited from water. Hutton's Theory may be stated shortly, as follows. The materials which compose the present surface of the globe, have been derived from the ruin of ancient rocks, which have been disintegrated and pulverized by the continued action of torrents and currents of water; and by the same means these materials have been transported to the bottom of the ocean. Here they have been consolidated, partly by time, and partly by the pressure of the water, but chiefly by the effects of subterranean heat. By the same cause, more powerfully exerted, that is, by the expansive power of volcanic heat, the strata thus formed have been elevated from the bottom of the ocean, to occupy the situations under which they now appear. Thus the strata are thrown into different degrees of inclination to the horizon; or are broken and dislocated; or appear in nearly a vertical position, depending on the degree of force, or the point of its application. Sometimes, also, where the heat has been most intense, an entire fusion of the materials has been effected. The rocks which are not stratified, or not composed of layers, as granite, are supposed to have undergone complete fusion, while those which consist of layers, as mica slate, are supposed only to have been softened by the heat. The same disintegration, and corrosion, and the same transportation to the sea, is constantly going on with respect to the present rocks, so that finally these materials will again be restored to the sea, to be again raised above its surface by volcanic fire, as before; and as the present continents were formed by the destruction of ancient rocks, so future continents will be formed in their turn, by rocks now preparing for that purpose.-Brande's Outlines of Geology. It is generally acknowledged at the present day, that Hutton's theory will account for a much greater number of geological phenomena than Werner's. It is impossible, for instance, to account for the present situation of stratified rocks containing sea shells, unless we suppose, either that the sea occupied the earth for ages, or that these strata were formed under the ocean, and elevated by some mighty force; and as we know that islands are thrown up from the sea, by volcanic force, at the present day, it is reasonable to attribute the same effect to the same cause, anciently. GENERAL FORM AND CONSTITUTION OF THE GLOBE. Although in a popular sense the form of the earth is that of a globe, yet science has long since determined, that its figure is not that of a perfect sphere, but of an oblate spheriod, the diameter at the equator being greater than at the poles. This difference has been variously estimated, but if we consider the polar, to the equitorial diameter in the proportion of 304 to 305, we shall perhaps come as near the truth as the present state of observations will allow. This allows the poles a compression equal to 35, and the two diameters as follows. Equitorial diameter, about 7,924 miles. 7,898 miles. Difference. 26 miles. This estimate is from the authority of Daubuisson. Dr. Macculloch makes the difference somewhat greater, but we need not here go into comparisons on a point where no two authors agree, the difference of a few miles being a matter of no consequence to our present purpose. This form of the earth is precisely such a one as it would have taken had it been a homogenous semi-fluid with a rapid motion round its axis. This form may be illustrated by fixing a ball of soft clay on a spindle and setting it in motion. The ball will be flattened at the poles or axis of motion, and elongated, or thrown out, at the circumference, or equator. This is obviously the consequence of the greater centrifugal force at the circumference than at the centre of motion. Density of the Earth. It has been attempted to estimate the density of the whole earth from that of a particular mountain. For this purpose Dr. Maskelyne made an experiment on mount Schihallien, a high precipitous cliff, in order to ascertain the force of its attraction, on a suspended plummet; with the view of deducing the density of the whole earth by comparing its attraction with that of the mountain. From such data it was found that the mean density of the earth was to that of the mountain as 9 to 5; and from hence it was concluded that the interior of the globe must be composed of substances whose density was about double that of the mass of the mountain. But it was subsequently found that the specific gravity of the mountain, an item in these calculations, had been estimated too high, and by the corrections made by Mr. Playfair, the density of the earth obtained in this way was found to be only 5. That is, the whole earth, bulk for bulk, is five times the weight of water. This estimate, which is most generally received at the present day, makes it necessary to suppose that the interior of the earth is much more dense than its surface; for the heaviest rock with which we are acquainted has a density of only 3, and the lightest about 24,while the specific gravity of the ocean is a little more than 1. The specific gravity of the earth's surface including the water, therefore, cannot be much above 24. But the mean density of the whole earth being 5, is more than double that of its surface, and hence the interior of the earth must have a greater density than 5, to counterbalance this want of weight at the surface. From these considerations, it has been supposed that the interior of the earth, instead of being composed of rocks, like the surface, must consist of metallic substances. It has also been shown from mathematical calculations that there is a gradual increase of density towards the centre of the earth, and hence it has been thought still more probable that its nucleus is of a metallic nature. La Place, with this view of the earth's structure, has estimated its density at the centre. If 5, 4 be taken as its mean density and its superficial densities be assumed as 3, 13; 3, 2; 2, 79; and 2, 60, then on the theory of the compressibility, the density at the centre will be 13, 25; 14, 54; 15, 78; and 20, 10, respectively. The least of these is about double the density of iron, and the greatest exceeds that of gold, being about equal to that of hammered platina, the most ponderous of all known substances. But both philosophy and conjecture are alike useless on this subject, for in spite of both, we must remain ignorant concerning the composition of the earth's centre. Distribution of Sea and Land. Nearly three fourths of the whole surface of the globe is covered by water. The surface of the Pacific Ocean alone is estimated to be somewhat greater than all the dry land with which we are acquainted.-(Daubuisson.) The greatest elevation of land is about 25,000 feet above the level of the sea; but its greatest depression, being concealed by the water, cannot be ascertained, and hence the quantity of water which the oceans contain cannot be estimated with any degree of accuracy. La Place, however, has made a computation of the mean depth of the sea, founded on the theory of the tides, by which he concludes that it is about twelve miles. Concerning the bottom of the sea, we know little more than that it consists of mountains and valleys, like the surface of the land. This is ascertained by the rocks, and islands which rise above the surface of the water, and the reefs and deep water, which are known to exist alternately, below it. The sounding line of the mariner not only detects this unevenness of the submarine surface, but also the steep acclivities of its mountains and the gradual risings of its sand banks. Composition and specific gravity of the Sea. The whole ocean is composed of salt water, though it varies considerably with respect to the quantity of solid matter it contains. At the mouths of rivers, and in bays which receive streams from the land, it is obvious that the water cannot be so fully saturated with salt, as it is where it is not thus diluted. There is also a difference wlth respect to the quantity of salt which different oceans contain, independently of any such circumstance. This is ascertained, not only by the analysis of their waters, but also by their different specific gravities. It will be remembered that the standard, or unity, by which specific gravities are estimated, is distilled water, which is 1; and therefore the greater the quantity of solid matter any water contains, the greater will be its specific gravity. Dr. Marcet instituted a series of experiments on sea water from different parts of the world, from which he ob tained the following results and conclusions. 1. "That the Southern Ocean contains more salt than the Northern, in the ratio of 1.02919 to 1.02757." 2. "That the mean specific gravity of sea-water, near |