606 DYNAMICAL GEOLOGY. 



pheric and oceanic circulation. The general facts on these topics are 

 briefly stated on pp. 38-46, which may well be reviewed before pro- 

 ceeding with the following pages. In treatises on Physical Geogra- 

 phy, these subjects may be studied at greater length, by the geological 

 student, with much advantage. 



The subject of dynamical geology is here treated under the follow- 

 ing heads : — 



1. Life ; 2. Cohesive and Capillary attraction ; 3. The Atmosphere; 

 4. Water ; 5. Heat ; 6. Consequences of the earth's cooling, and the 

 Evolution of the general features of the globe ; 7. In recapitulation, 

 Effects referred to their Causes. 



The chemistry of rocks, or the chemical processes concerned in their 

 origin and metamorphism, embracing a consideration of Life, the At- 

 mosphere, Water, Light, and Heat as chemical agents, would naturally 

 constitute another section, under the title of Chemical Geology. But, 

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

 and its study a minute knowledge of the principles of Chemistry, the 

 subject is not taken up in detail in this Manual. Some of the more 

 common facts are mentioned, under the head of Water as a chemical 

 agent (p. 687). 



I. LIFE. 



1. Protective Effects. 



The protective effects of life come chiefly 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 protec- 

 tion 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 

 tropical regions, plants growing at the water's edge, like the man- 

 grove, drop new roots from the branches into the shallow water, which 

 act like a thicket of brush-wood, to retain the floating leaves, stems, 

 and detritus ; and, as the water shallows, other roots are dropped 

 farther out, which are attended with the same effect ; and thus they 

 keep moving outward, and subserve the double purpose of protecting 

 and making land. The coarse salt-marsh grasses along sea-shores per- 



