INORGANIC GEOLOGICAL DYNAMICS. 549 



sea bottom while rock-bearing ice-masses floated on the surface till 

 they deposited their lading. 



Sir R. Murchison has pointed out another operation of ice in pro- 

 ducing -mounds of rocky masses; namely, the effects of rivers and 

 lakes, in climates where, as in Russia, the waters carry rocky frag^ 

 ments entangled in the winter ice, and leave them in heaps at the 

 highest level which the waters attain. 



The extent to which the effects of glaciers, now vanished, are appa- 

 rent in many places, especially in Switzerland and in England, and 

 other phenomena of the like tendency, have led some of the most 

 eminent geologists to the conviction that, anterior to the period of our 

 present temperature, there was a Glacial Period, at which the tem- 

 perature of Europe was lower than it now is.] 



Although the study of the common operations of water may give 

 the geologist such an acquaintance with the laws of his subject as may 

 much aid his judgment respecting the extent to which such effects 

 may proceed, a long course of observation and thought must be 

 requisite before such operations can be analysed into their fundamental 

 principles, and become the subjects of calculation, or of rigorous rea- 

 soning in any manner which is as precise and certain as calculation. 

 Various portions of Hydraulics have an important bearing upon these 

 subjects, including some researches which have been pursued with no 

 small labor by engineers and mathematicians ; as the effects of cur- 

 rents and waves, the laws of tides and of rivers, and many similar 

 problems. In truth, however, such subjects have not hitherto been 

 treated by mathematicians with much success ; and probably several 

 generations must elapse before this portion of geological dynamics can 

 become an exact science. 



Sect. 3. Igneous Causes of Change. Motions of the Earth's 



Surface. 



THE effects of volcanoes have long been noted as important and strik 

 ing features in the physical history of our globe ; and the probability 

 of their connexion with many geological phenomena, had not escaped 

 notice at an early period. But it was not till more recent times, that 

 the full import of these phenomena was apprehended. The person 

 who first looked at such operations with that commanding general 

 view which showed their extensive connexion with physical geology 

 tt'as Alexander von Humboklt, who explored the volcanic phenomena 



