394 WILL SCOTT. 



The Source of tJie Cave Plankton. To understand the dis- 

 tribution of the cave plankton, it is necessary to understand its 

 source. It has been assumed that the plankton (at least in part) 

 inhabiting a cave stream has been derived from the surface 

 through sink-holes. 1 An examination of the local sink-holes 

 made it clear that probably only a small part of the total num- 

 ber contribute. 



Sink-holes may be divided into two principal classes, com- 

 pound sink-holes and simple sink-holes. A compound sink-hole 

 is a large depression having secondary sinks on its slopes. These 

 arise in most cases probably by the reduction of the divides of 

 the compound sinks by erosion. Sinks may arise secondarily 

 on the slope of an old one. 



A simple sink-hole is one without secondary sinks. Simple 

 sink-holes agree in having the form of an inverted cone and are 

 of three types. The first type has an opening at the apex of the 

 cone leading to an underground passage. 



The second type has this opening closed. This closure is 

 inaugurated by rocks and earth caving in from the sides of the 

 opening. Sediment composed of the fine clay from the sides is 

 then deposited over this, and forms a very impermeable layer. 

 This results in a more or less permanent pond. 



The third type of simple sink-holes is like the second, except 

 that a new opening to a subterranean passage has been formed 

 on its sides a short distance above its lowest point. This new 

 opening is formed from below, but the details of the process I 

 could not determine. The result is a pond, which at its higher 

 levels overflows into the new opening. 



Open sink-holes are the rule in land covered with timber, be- 

 cause the erosion in such areas occurs slowly. 



The plankton is derived from the simple sink-holes of the third 

 type and compound sink-holes in which the lower sinks are open 

 and the upper are closed, and in which the divides are low 

 enough to allow the upper ones to overflow into the lower. The 

 first type of simple sink-holes'allows the water to flow so quickly 

 into the underground channel, that organisms do not have time 



1 The cave region is a plateau which has not yet developed surface streams. The 

 surface of this region is drained by funnel-shaped depressions called " sink-holes." 



