83 
smaller the percentage of pore space, and the thinner the beds the greater 
is the tendency to form sinks and caves and the more sharply angular are 
the subterranean drainage courses. ‘This is excellently illustrated in the 
cave region of Indiana. The Mitchell limestone is very dense, thin bedded 
and impervious for a limestone and is broken into small joint blocks. Here 
the subterranean flow is largely confined to the joints and bedding planes, 
thus concentrating the solution effected by the water to the immediate 
channels through which it flows. In this way channeis are produced and 
enlarged with maximum rapidity. 
On the other hand we may contrast this condition with that of the 
Salem limestone lying immediately beneath the Mitchell limestone. The 
Salem is nearly devoid or bedding planes, a rather soft and quite porous 
limestone, through which the water percolates with relative ease The re- 
suit is that caves in the Salem limestone are very rare. When they occur, 
Fig. 1. Diagrammatic illustration of incipient subterranean drainaze. The 
main stream is entrenched and the tributaries out of adjustment pitch over rapids 
te join it. Underground drainage has started through the joints. The vertical 
dotted line a b represents the original unbalanced static water head which started 
the circulation. 
as at Mays cave, they may be formed by a eave passing down from the 
Mitchell limestone inte it to reach the surface nearer the drainage level. 
The lack of frequent bedding planes is a strong contributory factor to this 
condition. Aside from its structure the opportunities for the formation of 
subterranean channels are as good as in the Mitchell limestone. 
Again, the Harrodsburg limestone, lying immediately below the Salem 
limestone, is harder, less porous, more highly jointed and thinner bedded 
than the Salem and shows a correspondingly greater tendency to develop 
underground water channels. The Mitchell limestone possesses the ex- 
treme of these conditions and the extreme development of underground 
channels. 
IHE SUBTERRANEAN. DRAINAGE CYCLE. 
In either a coastal plain or an interior region which has been thor- 
oughly baseleyeled and reéleyated, what drainage there is to begin with is 
