WORK OF GROUND-WATER 



39 





i u-k , since each mineral acts as a hinder for the rest. It may happen 

 that no one of the minerals is dissolved completely, but that one or 

 more of its constitutents is removed. Such change may cause the 

 mineral to crumble, and so destroy the integrity of the rock. These 

 are phases of weathering. 



Caverns. 1 In formations like limestone, which are relatively 

 soluble, considerable quantities of material may be dissolved from 

 a i:iven place. Instead of making the rock porous, in the usual 

 sense of the term, caverns are developed (Fig. 23). In their pro- 

 duction, solution may be abetted by the mechanical action of the 

 water passing through the openings which solution has developed. 

 Caves are numerous in central Kentucky and southern Indiana, 

 and the size of some of 

 them, such as Mam- 

 moth and Wyandotte, 

 is very great. A 

 ground-plan of Wyan- 

 dotte (Ind.) Cave is 

 shown in Fig. 24. The 

 aggregate length of its 

 passageways is a num- 

 ber of miles. 



Deposition may take 



place in caves after they are formed (Fig. 25), or it may even go on 

 at the same time that the cave is being excavated. Stalactites and 

 stalagmites are common forms of cave deposits. A stalactite may 

 start from a drop of water leaking through the roof of the cave. 

 Evaporation, or the escape of gases in solution, results in the deposi- 

 tion of some of the lime carbonate about the margin of the drop, in 

 the form of a ring. Successive drops make successive deposits on 

 the lower edge of the ring, which 'grows downward into a hollow 

 tube through which descending water passes, making its chief de- 

 posits at the end. Deposition in the tube ultimately may close it, 

 while deposition on the outside, due to the water trickling down in 

 that position enlarges it. 



Limestone sinks. Underground caves give rise to topographic 

 features of local importance. If the roof of a cavern collapses, it 

 causes a sink or depression in the surface. Some regions of lime- 

 stone caves are affected by numerous sinks formed in this way. 



1 For a racy account of caverns see Shaler's Aspects of the Earth. 



Fig. 24. Ground-plan of Wyandotte Cave. 

 The unshaded areas represent the passageways, 

 (aist Ann. Kept., Ind. Geol. Surv.) 



