EFFECTS OF WATER. 



44. 6th. To these may be added scoriae in tears, irregular 

 stala'ctites scattered on the surface of volcanoes, and volcanic 

 bombs, which are sometimes found at considerable distances. 



45. Volcanoes furnish annually but a small quantity of materials 

 to the solid crust of the globe, and the upheavals they cause pro- 

 duce very slight change in the elevation of countries where their 

 action is manifest. Nevertheless, if we remember that a great 

 number have been in action since the time of history, and observa- 

 tion shows that a great many more were previously in action, we 

 are led to the conclusion that volcanic substances are important, 

 and their presence must have occasioned great modifications on 

 the surface of our planet. 



LESSON VII. 



INFLUENCE OF EXTERNAL AGENTS ON THE SURFACE OF THE EARTH. 



Effects of the Atmosphere Degradation Effects of Winds 

 Dunes Effects of Lightning. 



EFFECTS OF WATER. Dissolving power Softening power 

 Denudation Erosion Effects of weight of Water Running 

 Waters Debacle of Lakes Mud-torrents Slope of Torrents 

 and Rivers Rolled Flints Transportation by Ice and Gla- 

 ciers Action of Waves Deposits formed by Water Geysers 

 Structure of sedimentary Deposits 7'a'lus Effects of 

 Transport or Drift Effects of oscillation in Waters Nature 

 of Deposits from Water Coral Reefs Polypa'ria Peat- 

 bogs. 



1. Atmospheric Effects. Variations of temperature, the air, 

 winds, dryness, and moisture, act very perceptibly on most mine- 

 ral substances ; there is not a rock on the surface of the earth which 

 does not present an appearance, externally, totally differing from 

 what is seen internally, when it is broken. This is everywhere 

 seen in escarpments formed by making roads, in mountainous 

 countries, where it is necessary to cut through rocks ; the exterior 

 is discoloured, and more or less extensively disaggregated, com- 



These effects are not 

 a few years are sufficient 

 only on the surface, but to considerable 

 depths : these effects are seen in ancient quarries of marble, or of 



44. What other solids are produced by volcanoes ? 



45. What influence do volcanoes exert on the elevation of countries ? 



1. How are the effects of the atmosphere on rocks manifested? How 

 does frost act on rocks ? Is a very long period of time necessary for the 

 atmosphere to produce its effects on rocks? 



