Descrizione:
By the earth's crust, is meant that small portion of the exterior of our planet which is accessible to human observation. It comprises not merely all of which the structure is laid open in mountain precipices, or in cliffs overhanging the river or the sea, or whatever the miner may reveal in artificial excavations; but the whole of that outer covering of the planet on which we are enabled to reason by observations made at or near the sur face. These reasonings may extend to a depth of several miles, perhaps ten miles; but even then it may be said, that such a thickness is no more than I%3th part of the distance from the surface to the centre. The remark is just; but although the dimensions of such a crust are, in truth, insignificant when com pared to the entire globe, yet they are vast and of magnificent extent in relation to man, and to the organic beings which peo ple 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. |
