. Elements of geology : a text-book for colleges and for the general reader. Geology. 166 GENERAL FORM AND STRUCTURE OF THE EARTH. 3.—The Crust of the Earth. The surface of the earth undoubtedly differs greatly in many re- spects from its interior, and therefore the exterior portion may very properly be termed a crust. It is a cool crust, covering an incandescent interior; a stratified crust, covering an unstratified interior; probably an oxidized crust, covering an unoxidized interior; and many suppose a solid crust, covering a liquid interior. This last idea, although very doubtful (p. 79),
. Elements of geology : a text-book for colleges and for the general reader. Geology. 166 GENERAL FORM AND STRUCTURE OF THE EARTH. 3.—The Crust of the Earth. The surface of the earth undoubtedly differs greatly in many re- spects from its interior, and therefore the exterior portion may very properly be termed a crust. It is a cool crust, covering an incandescent interior; a stratified crust, covering an unstratified interior; probably an oxidized crust, covering an unoxidized interior; and many suppose a solid crust, covering a liquid interior. This last idea, although very doubtful (p. 79), has probably given rise to the term crust. The term, however, is used by all geologists, without reference to any the- ory of interior condition, and only to express that portion of the ex- terior which is subject to human observation. The thickness which is exposed to inspection is about ten to twenty miles. Means of Geological Observation.—The means by which we are enabled to inspect the earth below its immediate surface are: 1. Arti- ficial sections, such as mines, artesian wells, etc. These, however, rarely penetrate below the insignificant depth of half a mile. 2. Natural sections, such as cliffs, ravines, canons, etc. These, as we have already seen (p. 17), sometimes penetrate 5,000 to 6,000 feet. 3. Folding, and subsequent erosion of the crust, by which strata from great depths have their edges exposed. Thus, in passing along the surface from s to a ( Fig. 131. 131), lower and lower rocks are successively brought under inspection. The dotted lines show how much has been cut away, and therefore the depth of strata exposed. In this way often ten miles depth of strata are brought into view. This is by far the most important means of ob- servation ; without it the study of geology would be almost impossible. 4. Volcanoes bring up to the surface materials from unknown but probably very great depths. Ten miles seems an insignificant fraction of the earth's radius, bei
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