CHAPTER III. THE GEOLOGICAL CONDITION OF THE EARTH. IN the preceding brief description of the Solar system, we see distinctly the relation which our own Earth bears to the other planets, in its position, its form, its magnitude, its satellite, and its daily and annual motions. But though a comparison of these properties of the earth, which constitute what may be called its astronomical condition, with the analogous properties of the other planets, might entitle us to ascribe to them other functions, the function, for example, of supporting inhabitants, which the earth only is known to possess, yet our argument will derive new strength, and we shall be prepared to meet recent objections, by taking into consideration the geological structure of the earth, and the properties of its atmosphere, and by endeavouring to read its past history in the successive steps by which it has been prepared as a residence for the human family. The earth, as we have seen, when merely examined by the eye, consists of land and water. The land is composed of soils of various kinds, and of stones and rocks of different characters. It is formed into extensive plains, into valleys excavated apparently by rivers or water-courses, and into mountain groups and mountain ranges, rising to the height of several miles above the bed of the ocean. In order to obtain a knowledge of the structure of the earth, geologists have examined with the greatest care its soils and its rocks, wherever they have been laid bare by natural or artificial causes, by the operation of the miner, or the road engineer, or by the action of rivers or of the sea; and they have thus obtained certain general results which give us an approximate idea of the different materials which compose what is called the crust of the earth. In those portions of its surface, which do not rise into mountains, the thickness of the crust thus explored does not exceed ten miles, which is only the 800th part of the earth's diameter,-a quantity so small, that if we represent the earth by a sphere having the same diameter as the cupola of St. Paul's, which is 140 feet, the thickness of the crust would be only about two inches. Beneath the crust lies the Nucleus of the earth, or its kernel or its skeleton frame, of the nature and composition of which we are entirely ignorant. We know only, by comparing the average density of the earth, which is about 53 times that of water, with the average density of the rocks near its surface, which is about 21 times that of water, that the density of the nucleus, if of uniform solidity, must exceed 51, and must be much greater if it is hollow or contains large cavities. Geology does not pretend to give us any information respecting the process by which the nucleus of the earth was formed. Some speculative astronomers indeed have presumptuously embarked in such an inquiry; but there is not a trace of evidence that the solid nucleus of the globe was formed by secondary causes, such as the aggregation of attenuated matter diffused through space; and the nebular theory, as it has been called, though maintained by a few distinguished names, has, we think, been overturned by arguments that have never been answered. Sir Isaac Newton, in his four celebrated letters to Dr. Bentley, has demonstrated that the planets of the solar system could not have been thus formed, and put in motion round a central sun. But though geologists have not been able to give us any intelligence respecting the earth's nucleus, they have examined the rocks which rest upon it, or the lowest of the series which extend upwards to the surface of the earth. I. The lowest of these rocks are granite, granitic rocks, trap, and porphyry. They are composed chiefly of the simple minerals, Quartz, Feldspar, Mica, and Hornblende. They are consequently crystalline and unstratified, and are believed to be of igneous origin. The next series of rocks are what are called the Metamorphic or altered rocks. They consist of gneiss, mica slate, and clay slate, and are more or less stratified. The next series of rocks is Basalt, or ancient lava, and what are called Trachytic Rocks. To these rocks the name of Primary has |