102 
on the map. There is anether case of piracy in the sinks near Kirksville 
whieh is of the same type as that just described. 
Other forms of piracy are probably much more common than the type 
described. That piracy occurs between adjacent subterranean 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 by the fact that cave streams are below the level 
of the general water table and also because the falling of slabs from the 
roofs frequently clog the channels and temporarily fill 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- 
plored with this object in view and because such cases will probably be 
ditlicult to recognize even under the most favorable circumstances. 
Cases of subterranean self-capture, capture of one tributary by an- 
other, or by the main stre;m 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 Hovey’s map of Mammoth cave is suffi- 
cient to suggest a mest complex and interesting set of captures and changes 
of level for some one to work out. 
RESUME. 
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 ifs easy access to drainage lines con- 
siderably below the generai 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. 
3. In regions of low elevation the sinks may be largely collapse 
sinks, and, in soft, porous rocks, the channels rather irregular. 
4. The eyele of underground drainage may be stated as follows: It 
begins with surface drainage and in its youth develops subterranean drain- 
nge near the points of easy escape for the water. In its maturity there is 
the maximum of subterranean drainage and the lower parts of the caverns 
have begun to retreat by collapse while in the uppermost reaches of the 
stream the transforimation from surface to subsurface drainage may still 
be in progress. Old age is shown by the more general condition of eol- 
