Oxygen is necessary to combustion, to the respiration of animals, and to various other natural operations, by all of which that gas is withdrawn from the air. It is obvious that its quantity would gradually diminish, unless the tendency of these causes were counteracted by some compensating process. This, to some considerable extent, is accomplished by vegetation, as it is found that healthy plants, under the influence of the sun's light, constantly draw carbonic acid from the air, the carbon of which is retained, while the oxygen is returned. The atmosphere becomes less and less dense from the surface of the earth upwards. Animals and vegetables exist in this atmosphere. They cannot exist in any other. All living things and beings live on this earth's crust. Vegetables are fixed to the soil of this earth, while animals move freely upon it. The earth's crust.--Sir Charles Lyell speaking of this earth's crust says: "By the 'earth's crust' is meant that small portion of the surface of our planet which is accessible to human observation, or on which we are enabled to reason by observations made at or near the surface. These reasonings may extend to a depth of several miles, perhaps ten miles; and even then it may be said that such a thickness is no more than 1/400 part of the distance from the surface to the center. The remark is just, but although the dimensions of such a crust are, in truth, insignificant when compared with the entire globe, yet they are vast and of magnificent extent in relation to man and to the other organic beings which people our globe. Referring to this standard of magnitude, the geologist may admire the ample limits of his domain, and admit at the same time that not only the exterior of the planet, but the entire earth, is but an atom in the midst of the countless worlds surveyed by the astronomer. "The solid part of this earth consists of distinct substances, such as clay, chalk, sand, limestone, coal, slate, granite, and the like. It has been imagined that the various deposits on the earth's surface were created in their present form and in their present position. On the contrary, it has been shown that they have acquired their actual configuration and condition gradually, under a variety of circumstances, and at successive periods, during each of which distinct races of living beings have flourished on the land and in the waters, the remains of these creatures still lying buried in the crust of the earth. "The materials of this crust are not thrown together confusedly; but distinct mineral masses called rock are found to occupy definite spaces and to exhibit a certain order of arrangement. These rocks are divided into four great classes by reference to their different origin, or in other words by reference to the different circumstances and causes by which they have been produced.