604 DYNAMICAL GEOLOGY. 



of atmospheric and oceanic circulation. The general facts on these 

 topics are briefly stated on pp. 39-48, which may well be reviewed 

 before proceeding with the following pages. In treatises on Physi- 

 cal Geography these subjects may be studied to greater length by 

 the geological student with much advantage. 



The subject of dynamics, or the causes or agencies in geological 

 history, is here treated under the following heads : — 



1. Life as an agent in protecting, destroying, and making rocks. 



2. Cohesive attraction, with reference especially to crystallization 

 and the concretionary structure. 



3. The Atmosphere, as a mechanical agent. 



4. Water, as a mechanical agent. 



5. Heat, as an agent in producing volcanic phenomena, non-vol- 

 canic igneous eruptions, metamorphism, veins, etc. 



6. Movements in the earth's crust, and their consequences, 

 including the plication of strata, origin of mountains, earthquakes, 

 and the evolution of the general features of the globe. 



7. The chemistry of rocks, or the chemical processes concerned 

 in their origin and metamorphism, embracing a consideration of 

 Life, the Atmosphere, Water and Heat as chemical agents. This 

 department of the science is often called Chemical Geology. As 

 its proper elucidation would require a large amount of space, and 

 its study a minute knowledge of the details of Chemistry, the sub- 

 ject is not taken up in this Manual. 



I. LIFE. 



1. Protective Effects. 



The protective effects of life come almost solely from vegetation. 



1. Turf protects earthy slopes from the wearing action of rills 

 that would gully out a bare surface ; and even hard rocks receive 

 protection in the same way. 



2. Tufts of grass and other plants over sand-hills, as on sea-shores, 

 bind down the moving sands. 



3. Lines of vegetation along the banks of streams prevent wear 

 during freshets. When the vegetation consists of shrubs or trees, 

 the stems and trunks entangle and detain detritus and floating 

 wood, and serve to increase the height of the margin of the 

 stream. 



4. Vegetation on the borders of a pond or bay serves in a similar 

 manner as a protection against the feebler wave-action. In many 



