FORM AND STRUCTURE OF THE EARTH. 175 



liquid which gave origin to the term crust ; but the word 

 is now used only to signify the superficial portions of the 

 earth, subject to human observation, without any impli- 

 cation as to the interior condition. 



Means of Geological Observation. As thus defined, 

 the crust is estimated at from ten to twenty miles in 

 thickness. The manner in which we get a knowledge of 

 the earth to that depth, or the means of geological obser- 

 vation, are 1. By mines and artesian wells. These pene- 

 trate 4,000 or 5,000 feet. 2. Canons and ravines. These 

 give sections of 6,000 or 7,000 feet. 3. Volcanic ejections. 



FIG. 89. 



These bring up matter from unknown but certainly still 

 greater depth. But the most common and effective means 

 of observation is furnished by 4. Foldings of the crust, 

 and subsequent erosion. In the section (Fig. 89) in which 

 s s is the present surface, we represent one of the com- 

 monest of all geological phenomena. It is seen that 

 from the point a the strata are repeated on the two sides. 

 The dotted lines show how much has been cut away, and 

 what depth of strata has been exposed to view. In this 

 way, in very many places, the character of the rocks ten 

 or more miles deep is revealed. 



Our direct observation is absolutely confined to this 

 superficial portion. We can only speculate about what is 

 beneath. It would seem, at first sight, that this is an 



