ORIGIN OF CAVERNS. 



Many great rivers have themselves 

 cut their beds in the ancient allu'- 

 vium (Jig. 273), very different from 

 that now forming; the Seine, at Pa- 

 ris, excavated its bed in a deposit 

 of rolled flints very unlike the gravel Fig. ^73. Vailey <>f erosion in 6 



it now deposits. moioe.able formation. 



20. Valleys from disruption, are those which have been pro- 

 duced by cracks of every size, sometimes colossal, during the up- 

 heavals that have brought the land to its present configuration of 

 surface. They generally present abrupt escarpments, in which 

 are seen the section of the fractured strata, the projecting angles 

 on one side often corresponding with the retreating angles of the 

 other. The circles which frequently terminate them above, or 

 those that divide them in their length, are so many craters of ele- 

 vation, most of which are clearly characterized either by the up- 

 lifted strata or the barrancos they present. 



21. Valleys of subsidence are also spoken of, but it does not 

 appear there are any arising purely from this cause. Subsidence 

 is frequently correlative to upheaval ; and valleys as well as craters 

 of elevation may exhibit the effects of both, which must have taken 

 place especially in the circles found along their line, and at their 

 superior extremity. 



22. Valleys from folding or plaiting are produced by two 

 neighbouring upheavals, causing the elevation of strata, and leaving 

 a space between, the slopes of which being formed by their planes; 

 this is seen in the high parts of the Jura (Jig. 269.) Many rivers 

 flow in valleys resulting from two opposite uptiltings of the soil. 



23. Valleys of erosion or denudation are produced in loose 

 formations like ravines, made by rain-storms, the waters of which 

 carry off the materials constituting the soil. 



24. The origin of caverns is one of the phenomena attributed 

 to the action of water ; but, although we find on a level with the 

 sea some caverns of slight depth, which may have arisen from the 

 repeated action of waves, it is difficult to believe that great caves, 

 which are sometimes many leagues in extent, have been produced 

 solely by the action of the waters running through them. The 

 action of water on compact limestone, in which caves are princi- 

 pally found, is so slight, that it has been supposed the open spaces 

 now found, were at one time filled by masses of salt, which the 

 waters had subsequently dissolved and carried away. 



It is presumed, however, that the first origin of caverns is due io cracks, 

 produced in the interior of the soil, which have been afterwards modified by 



20. What are valleys of disruption? 



21. What are valleys of subsidence? 



22. How are valleys from folding- produced? 



23. How are valleys of denudation formed in loose strata ? 

 *24. How is the origin of caverns accounted for ? 



