136 GEOLOGY. [chap. vi. 



after it visited Portugal." It may be questioned, however, 

 whether we have here an instance of the rapidity of undula- 

 tions, or rather of the simultaneous exertion of volcanic 

 forces, proportionally affecting at the same moment certain 

 weak points of the surface of the earth. Countries raised 

 by volcanic action so late as the tertiary period appear to 

 be still more subject than others to earthquakes, and Madeira 

 is not an exception to this rule. 



