THE PLANKTON OF SHAWNEE CAVE. 389 



Eight hundred feet below the " Big Room " is the fourth ob- 

 struction (1-13). The roof has fallen from some cause that I 

 was unable to determine, and has completely dammed the stream. 

 A new channel has been formed from this point to the outlet, 

 except for a distance of 40 feet, where it flows through an old 

 cave. The direct result of these obstructions is the formation of 

 pools having a maximum depth of about 10 feet. 



Elevation. The outlet of Shawnee Cave is 40 feet above the 

 level of White River which is about two and one half miles dis- 

 tant. The Lower Twin Cave is 30 feet above Shawnee Cave and 

 Upper Dalton Cave is 10 feet above Lower Twin Cave. The 

 gradient of the stream in the lower part of the cave is 40 feet to 

 the mile. It falls very rapidly from the outlet for about 800 

 feet, and then has a gradient of about 12 feet to the mile. Prob- 

 ably this slight gradient and the close approach of the stream to 

 local base level prevents the stream from finding a lower level 

 when obstructed, thus causing the effect of the obstruction to 

 continue. 



As a result of the pools at the turns and above the obstruc- 

 tions, the water in the cave may be divided into a constant which 

 is the amount of water in the pools at all times and a variable 

 which is the amount of water flowing through the cave. The 

 ratio between the constant and the variable is much greater in 

 the lower (ordinary) stages of water than in times of flood. At 

 ordinary stages of water, it requires a given particle of water 

 much longer to pass through the cave than if the cave were a 

 straight tube. 



Five hundred wooden cubes which had been soaked in paraf- 

 fine were put in at Lower Twin Cave, and a trap of " hardware " 

 netting (one half inch mesh) was set at Shawnee Cave. 

 Fourteen days later the first blocks were caught at Shawnee 

 Cave. In times of flood the current in very rapid. Just what 

 the rate is could not be determined as no trap could be designed 

 that would withstand the terrific force of the stream at Shawnee 

 Cave. Certainly not more than a few minutes are required for 

 water to pass from Twin Cave to Shawnee Cave in times of 

 flood. 



It is evident, then, that the plankton of the cave is subjected 



