PART IV. 



DYNAMICAL GEOLOGY. 



Dynamical Geology treats of the causes of events in the earth's 

 geological progress. 



These events include — the formation of all rocks, stratified and 

 unstratified, with whatever they contain, from the earliest Azoic to 

 the modern beds of gravel, sand, clays, and lavas ; the oscillations 

 of the earth's crust ; the increase of dry land, elevation of moun- 

 tains, and elimination of the surface-features of the globe ; the 

 changes of climate ; the changes of life. 



The causes or agencies, exclusive of life, that have been engaged 

 have acted for the most part through the atmosphere, waters, and 

 rock-material. But they are based necessarily on the general 

 powers of Nature, — Heat, Light, Electricity, and Attraction. These 

 fundamental powers have their universal laws, — as the law of 

 gravitation, according to which falling bodies move; the laws of 

 chemical attraction, according to which compounds are formed 

 and decompositions take place ; the laws of cohesion or crystalliza- 

 tion, according to which solidification produces crystals, or a crys- 

 talline structure ; the laws of heat, as regards conduction, expan- 

 sion, etc., and the influence of heat on chemical changes and 

 growth ; the laws of light, as to its nature, and its action in chemi- 

 cal changes and growth, etc. ; the laws of electricity and mag- 

 netism : all of which the geologist cannot understand too well. 

 But the discussion of these topics belongs properly to a treatise on 

 Physics. The laws of solidification are, however, briefly considered 

 in this place, on account of their bearing on the structure of rocks. 



In addition to the general operation of forces, there are other 

 actions, that may be embraced under the term climatological, which 

 proceed from the systematic arrangement and movement of heat, 

 light, moisture, and electricity about the sphere (causing zones of 

 temperature, varieties of climate, etc.), and also from the systems 



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