102 



on the map. There is aii'jther case of iiiraey in tiie sinks near Kirksville 

 which is of the samo type as that just dc'^eribed. 



Other forms of piracy are probably nnicli more common tliaii the type 

 described. Tluit piracy occurs bet\\een adjacent suliterranean streams 

 seems very probable on account of the greatly varying levels occupied by 

 them at different times and different parts of the same stream at the same 

 time. This is facilitated l)y the fact that cave streams are below the level 

 of the general water table and also because the falling of slabs from the 

 roofs frecpiently clog the clumnels and temporarily till the caves with water 

 until further underground passages may be discovered and enlarged. It is 

 impossible to cite specific cases at present because caves have not been ex- 

 rilored with this olijec-t in view and because such cases will probably be 

 dilficult to recognize even under the most favorable circumstances. 



Cases of subterranean self-capture, capture of one tributary by an- 

 other, or by the main stream or the capture of the main stream by a tribu- 

 tary finding a short cut through a new channel are too common to be dis- 

 cussed at length here. A glance at Ilorey's map of Mammoth cave is suffi- 

 cient to suggest a most complex and interesting set of captures and changes 

 of level for some one to work out. 



1. Extensive subterranean drainage is developed in interior regions 

 only when they have been sufficiently elevated to allow rapid downward 

 movement of the ground water and its easy access to drainage lines con- 

 siderably below the general level of the land surface. 



2. The conditions best facilitating subterranean drainage are regions 

 well elevated with relatively impervious soluble rocks, well jointed and 

 thinly bedded. 



;>. In regions of low elevation the sinks may be largely collajise 

 sinks, and, in soft, porous rocks, the channels rather irregular. 



4. The cycle of underground drainage may be stated as follows: Ir 

 begins with surface drainage and in its youth de\e!o])s subterranean drain- 

 age near the points of easy escajie for the water. In its maturity there is 

 the maxinuun of subt<M'ranean drainage and the lower i>arts of the caverns 

 have begiui to retreat by c(>l!a].>se while in the uppermost reaches of the 

 stream the transformation from surface to subsurface drainage may still 

 be in progress. Old age is shown by the niore general condition of col- 



