276 GEOLOGY. 



But in later times, when young, vigorous giants, such as 

 the Alps, the Himalayas, or, later still, the Rocky Mount- 

 ains, forced their way out from their fiery prison-house, 

 the crust of the earth was much thicker, and fearful in- 

 deed must have been the convulsions which attended 

 their exit." 



With the raising of so many mountains, the great sys- 

 tems of rivers alluded to in 165 were begun; but, as 

 they were not completed till the next age, I will defer 

 the consideration of this subject till I come to speak of 

 that age. 



386. Tertiary Volcanoes. There are evidences that 

 volcanic agencies were at work during the Tertiary age 

 in various parts of the earth. Such evidences have been 

 found in the West India islands, in Central America, in 

 parts of the Andes, and on the eastern continent in 

 France, Italy, Spain, Greece, and the island of Sicily. 

 The most interesting and striking evidences are found in 

 the ancient provinces of Auvergne, Velais, and Vivarais, 

 in the centre of France. Many hundreds of cones are 

 seen there, and the streams of lava which issued from 

 them can be traced, in many cases, as distinctly as those 

 which in this present age so recently have flowed from 

 Vesuvius or Etna. Some of these cones are represented 

 in Fig. 166. Many of them have been preserved with 



Fig. 166. 



remarkable distinctness. This is owing to their loose, 

 porous structure, which at first thought would be con- 

 sidered as rendering them very destructible. The ex- 

 planation is, that all the rain which falls is absorbed at 



