THE WORK OF GROUND-WATER. 



227 



more soluble than the others, that one may be dissolved. This has 

 the effect of breaking up the rock, since each mineral acts as a binder 

 for the rest. It might happen that no one of the minerals is dissolved 

 completely, but that some one of them is decomposed by water, and 

 certain of its constituents removed. Such 

 change would be likely to cause the mineral 

 so affected to crumble, and with its crumbling, 

 if it be an important constituent of the rock, 

 the integrity of the rock is destroyed. Where 

 considerable chemical changes, especially sub- 

 tractions, are going on, the rock is likely to 

 crumble. The increase in volume attendant on 

 hydration, etc., sometimes leads to the dis- 

 ruption of rock. These are phases of weather- 

 ing. (For other phases of weathering see 

 pp. 54 and 110.) 



Cavems.i — Where local solution is very 

 great results of another sort may be effected. 

 In formations like limestone, which are rela- 

 tively soluble, considerable quantities of mate- 

 rial are frequently dissolved from a given place. 

 Instead of making the rock porous, in the 

 usual sense of the term, large caverns may be 

 developed (Fig. 202). In their production, 

 solution may be abetted by the mechanical 

 action of the water passing through the open- 

 ings which solution has developed. Consider- 

 able caves are found chiefly in limestone. They 

 were probably developed when the surface 

 relief was slight, and surface drainage therefore 

 poor. Regions where caves were developed 

 under these conditions may subsequently acquire relief, so that caves 

 are not now confined to flat regions. 



One of the best known regions of caves is in the basin of the Ohio 

 in Kentucky and southern Indiana, where the number of caves is 

 large, and the size of some of them, such as Mammoth and Wyandotte, 



Fig. 203.— Ground-plan of 

 Wyandotte Cave. The 

 unshaded areas repre- 

 sent the passageways. 

 (21st Ann. Rept., Ind. 

 Geol. Surv.) 



* For a racy and interesting account of caverns see Shaler's Apects of the Earth 



