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. 



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, being 

 in fact equivalent to less than one thirtieth of an inch in a globe two 

 feet in diameter. It may seem at first sight an insufficient basis for a 

 science of the earth. We must recollect, however, that only this crust 

 has been inhabited by animals and plants — on this crust only have 



