THE WORK OF GROUND-WATER. 



229 



very great. A ground-plan of ^Yyandotte (Ind.) Cave is shoTVTi in 

 Fig. 203. The aggregate length of the passageways is about 23 J miles. 

 Deposition often takes place in caves after they are formed (Figs. 204 

 and 205). It may even go on at the same time that the cave is being 

 excavated. Here are formed the well-knowTi stalactites and stalag- 

 mites. A stalactite may start from a drop of water leaking through 

 the roof of the cave. Evaporation, or the escape of some of the car- 

 bonic gas in solution, results in the deposition of some of the lime car- 

 bonate 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 doTMiward into a hollow tube through which descending water 

 passes, making its chief deposits at the end. Deposition in the tube 

 may ultimately close it, while deposition on the outside, due to water 

 trickling dovn in that position, may greatly enlarge it. 



Fig. 206. — A limestone sink-hole, east-northeast of Cambria, Wyo., exceptional for 

 its steep sides. Minnekahta limestone. (Darton, U. S. Geol. Surv.) 



Underground caves sometimes give rise to topographic features 

 which are of local importance. AVhen the solution of material in a 

 cavern has gone so far that its roof becomes thin and weak, it may 

 collapse, giving rise to a sink or depression in the surface over the site 



