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 un- 

 stratified, with whatever they contain, from the earliest Archaean to 

 the modern beds of gravel, sand, clay, and lava ; the oscillations of the 

 earth's crust ; the increase of dry land, elevation of mountains, and 

 elimination of the surface-features of the globe ; the changes of cli- 

 mate ; the changes of life. 



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

 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 funda- 

 mental powers have their universal laws, — as the law of gravitation, 

 according to which falling bodies move ; the laws of chemical attrac- 

 tion, according to which compounds are formed and decompositions 

 take place ; the laws of cohesion or crystallization, according to which 

 solidification produces crystals, or a crystalline structure ; the laws of 

 heat, as regards conduction, expansion, etc., and the influence of heat 

 on chemical changes and growth ; the laws of light, as to its nature, 

 and its action in chemical changes and growth, etc. ; the laws of elec- 

 tricity and magnetism : 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 ac- 

 tions, 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 tempera- 

 ture, varieties of climate, etc.), and also from the systems of atmos- 



