128 



NEW YORK STATE MUSEUM 



DYNAMIC GEOLOGY 



Under this head there is only enough space to enumerate the 

 different agencies which are productive of geologic change or are 

 associated with it. For a detailed discussion the student is re- 

 ferred to the text-books. 



The dynamic agencies of geology may be roughly classified 

 into two groups : hypogene or subterranean and epigene or super- 

 ficial. Under the first head the principal agencies are volcanoes, 

 earthquakes, secular changes of level and metamorphism. 

 These, as their group name indicates, are chiefly controlled by 

 forces that work beneath the surface of the earth; the second 

 or epigene group comprises those which are chiefly manifest upon 

 the earth's surface. First among these is the air. Air in motion 

 or wind, is of marked importance as an agent of transportation 

 as manifested in sand dunes, at places where deposits of fine 

 sand occur, chiefly on the sea shore and in deserts. 



A more active agent than air is water. By the action of its 

 terrestrial forms, rain, snow and ice and by the cumulative forms 

 of these, rivers and glaciers, the highlands are reduced and vast 

 amounts of material are transported by the aid of gravity. 



The oceanic waters are agents of destruction, transportation 

 and formation. Waves beat upon the land and loosen fragments 

 from the rocks upon which they beat. These fragments, carried 

 out within reach of the oceanic currents, are borne along and 

 drop to the bottom forming sand bars and other sub-aqueous 

 deposits. Lastly, animal and plant life, both terrestrial and 

 aquatic, are formidable agents of change, both destructive and 

 constructive. 



Resume 



Hypogene or subterranean agencies 

 Volcanoes 



Earthquakes 



Secular change of level 



Metamorphism 



