606 HISTORY OF GEOLOGY. 



which have been produced, certainly or probably, 

 since the earth's surface assumed its present form, 

 we have still an ample exhibition of powerful causes 

 of change, in the streams of lava and other mate- 

 rials emitted in eruptions ; and still more in the 

 earthquakes which, as men easily satisfied them- 

 selves, are produced by the same causes as the 

 eruptions of volcanic fire. 



Mr. Ly ell's work was important in this as in 

 other portions of this subject. He extended the 

 conceptions previously entertained of the effects 

 which such causes may produce, not only by show- 

 ing how great these operations are historically 

 known to have been, and how constantly they are 

 going on, if we take into our survey the whole sur- 

 face of the earth ; but still more, by urging the con- 

 sequences which would follow in a long course of 

 time from the constant repetition of operations in 

 themselves of no extraordinary amount. A lava- 

 stream many miles long and wide, and several yards 

 deep, a subsidence or elevation of a portion of the 

 earth's surface of a few feet, are by no means extra- 

 ordinary facts. Let these operations, said Mr. Lyell, 

 be repeated thousands of times ; and we have results 

 of the same order with the changes which geology 

 discloses. 



The most mitigated earthquakes have, however, 

 a character of violence. But it has been thought 

 by many philosophers that there is evidence of a 

 change of level of the land in cases where none of 



