EFFECTS OF WATER. 125 



seaward drives the sand from the foot of the hillock (Jig. 207, ), 

 to its summit (6), whence it falls in the line b, c, forming at this 

 point a falling talus, always more abrupt than the first or rising 



Fig. 207. Fig. 208. 



Progress of dunes, or moving sands. 



*a'lus. The result of this is a single hillock, a b c, taken sepa- 

 rately (fig. 208), which grows behind, if new sands be furnished 

 in front, or it is displaced, if the same sands are continually re- 

 moved. Now, the wind acting on all these hillocks at the same 

 time, the mass formed by them is found to have moved a certain 

 distance inland, in a short time, while new heaps are formed in 

 front, at the expense of the sands freshly washed up from the sea. 

 It is calculated that dunes advance, in this way, twenty or thirty 

 yards a year; so that it is evident there must have been a time when 

 they were far from the places they have invaded. A great many 

 localities are known, which have been submerged by these seas of 

 sand. 



4. Lightning sometimes produces remarkable effects ; in a great 

 many places and on various rocks, traces effusion by thunderbolts 

 in high mountains have been observed. According to the observa- 

 tions of Friedler, when lightning penetrates sand, it often forms 

 narrow, irregular canals to a great depth, the sides of which are 

 consolidated by the fusion of quartz itself; and there are instances 

 where considerable portions of rocks have been turned round, torn 

 from their places and hurled to great distances by lightning. 



5. Effects of Water. Water plays a very important part in the 

 changes which are taking place on the surface of the globe ; some- 

 times by its dissolving power, but more frequently by its softening 

 action, its weight, and especially by the motion that may be com- 

 municated to it, and by the transporting power resulting from its 

 rapidity. The extent and importance of modifications from this 

 agent ought to be understood. 



6. Dissolving power. Water exerts a chemical action on some 

 substances which it dissolves, either directly or by means of the 

 cnrbonic acid it may contain. It acts directly on some salts which 

 it meets here and there, or on some deposits of sulphate of lime, 

 which it corrodes in various ways. When more or less charged 

 with carbonic acid it acts on calcareous rocks, either under ground 

 or where they crop out on the surface ; or in high mountains at the 

 time snows are melting. In this case, the water generally pos- 

 sesses itself of the carbonic acid contained in the air, in greater 



4. What are the effects of lightning on rocks? 

 5 By what properties does water produce its effects on rocks ? 

 6. What effects result from the dissolving power of water ? 

 11* 



