EFFECTS REFERRED TO THEIR CAUSES. 837 



VIII. EFFECTS REFERRED TO THEIR CAUSES. RE- 

 CAPITULATION. 



In many cases, the same effect — the formation of valleys, for ex- 

 ample — has come from different causes; and the subject is therefore 

 discussed in different places, in the course of the preceding pages on 

 Dynamical Geology. In this chapter, the pages are mentioned where 

 each topic is considered ; and under some subjects additional explana- 

 tions are introduced. 



I. Fkagmental Material or Deposits. 



1. Sources of Sand, Gravel, Stones. 



A. Mechanical. — 1. From erosion by water, pp. 648, 680. 



2. From erosion by means of winds, p. 632. 



3. From the abrasion of rocks or stones moved by ice, pp. 538, 699. 



4. From the abrasion of opposite walls of fractures. 



5. Through the freezing in the crevices of rocks, a very efficient agency in regions 

 of cold winters, p. 687. 



6. Through the divellent action of the growth of vegetation in crevices or fissures — 

 a work in which all kinds of plants serve, from Lichens and microscopic fresh-water 

 Algae to great trees, and which produces vast results, p. 607. 



7. Through the mutual attrition of rocks or stones in a slide, p. 665. 



8. Through ordinary changes of temperature, expanding and contracting the super- 

 ficial portion of a rock, p. 720. 



9. Through the explosion of bubbles of lava in a volcano, producing volcanic cinders, 

 and the material of tufas, p. 728. 



10. Through the tearing action of the ice of the under part of a glacier, p. 538. 



B. Chemical. (1) Through the chemical alteration or decomposition of one of the 

 essential or adventitious constituents of rocks, p. 703. 



(2.) Through the action of acid or alkaline solutions from some external source, 

 p. 705. 



2. Rounding of Stones, Making Bowlders. — (1.) Through the attrition 

 caused by moving waters, air, or ice. 



2. Through the loosening of surface-grains or outer layers in succession, by ordinary 

 alternations of surface temperature, the action from two directions at the edges, and 

 from three at the angles, ultimately producing curved surfaces, p. 720. 



3. By decomposition at the surface — a cause, that, like the last, removes the edges 

 and angles most rapidly, p. 87. 



4. By revolution in the air, on ejection to a considerable height from the throat of a 

 volcano, producing what are called volcanic bombs, p. 736. 



3. Assorting of Fragmental Material. — (1.) By variations in the rate of 

 flow of waters, p. 654. 



2. Through the unequal wear of harder and softer grains, under the action of the 

 waves or running water, the softer being worn first and drifted off, and so leaving the 

 harder behind, as in the making of a sand-beach, p. 655. 



3. By the action of the winds. 



4. Transportation of Fragmental Material. — 1. By fresh or salt water, 

 pp. 653, 677. 



2. By ordinary floating ice, icebergs, or glaciers, pp. 538, 698, 701. 



3. By the winds, pp. 631, 632. 



