THE EARTH'S SURFACE. 349' 



ocean surrounding it, teems with life. This state of 

 things lasts for ages ; but the time arrives when the 

 ocean again floods the land, and a new state of tilings, 

 over a particular district, has a beginning. 



It must not be imagined that the changes which wehave 

 spoken of, as if they were the result of slow decay and 

 gradual deposit, were effected without occasional violent 

 convulsions. Many of the strata which were evidently 

 deposited at the bottom of the sea, and, of course, as 

 horizontal beds, are now found nearly vertical. We 

 have evidence of strata of immense thickness having 

 been subjected to forces that have twisted and contorted 

 them in a most remarkable manner. Masses of solid 

 rock, many thousand feet deep, are frequently bent and 

 fractured throughout their whole extent. Mountains 

 have been upheaved by internal force, and immense 

 districts have suddenly sunk far below their usual level. 

 By the expansive force due to that temperature which 

 must be required to melt basaltic and trap rocks, the 

 whole of the superficial crust of a country has been 

 heaved to a great height, immense fissures have been 

 formed by the breaking of the mass, and the melted 

 matter has been forced through the opening, and over- 

 flowed extensive districts, or volcanoes have been formed, 

 and wide areas have been buried under the ashes ejected 

 from them. With the cause of these convulsions we 

 are at present unacquainted. 



We have evidence of the extent to which these forces 

 may be exerted, in the catastrophes which have occurred 

 within historical times, and which have happened even 

 in our own day. Herculaneum and Pompeii, buried 

 under the lava and ashes of Vesuvius, in an hour when 

 the inhabitants of these cities were unprepared for such 

 a fearful visitation, the frightful earthquakes which 

 have, from time to time, occurred in South America 

 are evidences of the existence of hidden forces which 

 shake the firm-set earth. Similar ravaging catastrophes 



