PAKT III. 



DYNAMICAL GEOLOGY. 



Dynamical Geology, as explained on page 14, treats of the causes of 

 events in the earth's geological progress. These events include : I. Those 

 concerned in the production and modification of the earth's rock structure, 

 and in the development of its form and features. II. The changes in the 

 earth's climates. III. The changes through geological time in the earth's 

 vegetable and animal life. The explanations beyond relate mainly to the 

 first of these classes of subjects. The succession in climates and in vege- 

 table and animal life is considered only historically, under Historical 

 Geology. 



The chief of the agencies directly concerned in geological work are the 

 Atmosphere, the Waters, Heat, Chemical Force, and Life, each acting through 

 or under general physical laws. 



The atmosphere and the waters, by means of which most rocks have been 

 made, valleys excavated, mountains shaped, and a great amount of chemical 

 work carried on, are the most prominent of the earth's exterior agencies. 

 Life, in its geological work, is another of the exterior agencies. Heat has 

 both an exterior and an interior source, with corresponding effects. As 

 exhibited in igneous ejections and volcanoes it is an interior agent both in 

 source of material and of force ; but the distribution of ejected material has 

 taken place in part by means of the exterior agencies, water and air. The 

 agencies that have made continents, oceanic depressions, and mountain ranges 

 are largely interior in the origin of their forces and in their work. 



There are three chief sources of energy for these agencies : — 



1. The earth's rotation on its axis, and its revolution around 

 THE SUN. (1) The rotation determining the earth's spheroidal shape, the 

 length and alternations of its day, its zones of climate, and the system of 

 movements in physical agencies ; (2) the revolution, causing, in case of col- 

 lision with any foreign body (as a meteorite), a manifestation of force in the 

 production of heat and in violent mechanical effects. 



2. The sun : which, through its heat, light, and attraction, is the origin 

 of movements in the air, oceans, and rivers ; the origin of chemical ac- 

 tivity and growth in the kingdoms of life, and of much chemical work in 



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