TABLE OF CONTENTS. A description of the more important minerals from the INTRODUCTION. THE ellipsoidal form of the globe, the temperature of its strata which is greater in proportion to the depth, the incandescence of the lavas thrown from volcanic craters, tend to establish the fundamental fact, that the earth has passed through a condition of igneous fusion. By cooling, a solid crust has been formed whose thickness has increased with the lapse of time. Among the mineral matters which enter into the composition of this solid crust, a certain number appear in considerable masses, and often with identical or very analogous characters in regions widely separated from each other. It is to these masses that we give the name of Rocks. The water which covers three-fourths of the surface of the globe, the air which surrounds it in all directions, the gases and vapors which escape from its depths, merit doubtless the name of elementary parts of the globe; but the determination of these elements of our planet is not conducted by the same process as that for the solid masses or rocks. Of the centre of the earth-the portion enveloped by the exterior solid portionwe know neither the composition nor the physical condition. It is quite certain that its temperature is very high; and it is known that the mean density of this interior portion is higher than that of the surface; but the composition is entirely conjectural. The rocks called gneiss, composed of crystalline elements disposed in their distinct layers, are regarded as being among those which resulted from the slow cooling of the incandescent globe. Many geologists add also certain fine-grained granites. Rocks of such origin are called primitive. A Immediately that the temperature of the surface was sufficiently lowered, the water condensed from the atmosphere, and charged with vapors or gases of different kinds, commenced its chemical role as a solvent, and its mechanical rôle as a motor, transperting to points already low, the débris and detritus torn from rocks in the more elevated regions. This débris is sometimes only the fragments of the former rocks; sand is an example of this kind. Sometimes it is composed of minerals to which the rocks, in decomposing, have contributed only a part of their substance, of which many calcareous rocks furnish examples. Rocks thus formed have oftentimes the appearance of the earth of our fields, and are, in consequence, called earthy. They are furthermore termed sedimentary, because they have been deposited by water; and stratified, because they are disposed in strata or successive parallel layers. There is a thirc, origin of rocks also very important. From the earliest epochs, there have been produced great rents in the earth's crust, into which have been injected, in a more or less pasty condition, the rocks termed eruptive, to which class belong the greater part of the Granites, Porphyries and Trachytes. To this class belong also the volcanic rocks, such as Basalts, Lavas, and the different solid matters which are at the present time thrown from the craters of volcanoes. Such are the three great orders of rocks, characterized by the following very different relations of position: (1) Rocks of crystalline structure which have apparently cooled from a fused condition, and of which the oldest are the uppermost and superposed upon the more recent; (2) sedimentary or earthy rocks, of which the first in the order of time are the lowest and are covered by the more modern; (3) eruptive rocks, which are further subdivided into volcanic rocks, and those which in a fused condition have been pushed through the outer covering of rocks already deposited above them. The eruptive rocks have their base in the profound depths of the globe, and connect it, at least at the epoch of their consolidation, to the concentric |