39 WILL SCOTT. 



to the cave environment for a much longer time during the lower 

 stages of water than during the higher stages. This has a marked 

 effect upon the number of species and the number of individuals 

 constituting the plankton at the different stream levels. 



Light. This cave is like most others for the greater part of 

 its length, in that there is an absence of light. It differs from 

 most others, in that the stream is illuminated at the points where 

 the roof has collapsed. The stream is fairly rapid in the sections 

 exposed to light, so that the illumination is of short duration. 

 The illumination is strong between Dalton caves and scarcely 

 more than twilight at low stages of the stream between Twin 

 caves. These short exposures to light may enable some of the 

 zooplanktonts to feed and cause some carbon assimilation in the 

 phytoplanktonts. The effect is probably very slight. 



Temperature, (a) Air. The temperature of the air of the inte- 

 rior of the cave is between 5256 F. throughout the year. The 

 temperature has been recorded for the last two years on a thermo- 

 graph stationed fourteen hundred feet below Lower Twin Cave. 

 The variation exceeds the error of the instrument slightly. 

 Where the air flows out at an opening it differs in observed 

 cases less than one (i) degree Centigrade from the temperature 

 of the interior. Inflowing air assumes the temperature of the air 

 in the center of the cave gradually. 



On September 7, 1908, a Centigrade thermometer carried into 

 Lower Twin Cave showed that the interior temperature was 

 reached 428 feet from the opening. During extreme tempera- 

 tures above ground this point would be farther from the opening. 



() Water. The water temperature varies much more than 

 that of the air for obvious reasons. During low water the tem- 

 perature of the water approaches the temperature of the walls 

 of the cave. During a flood, it varies toward the temperature 

 of the water outside the cave. Floods, then, cause the tempera- 

 ture of the cave water to lower in winter, to rise in summer, but 

 affect it slightly when the outer temperature is near 54 F. The 

 variation is much less in summer than in winter, because the 

 summer floods are not so great. 



In every flood observed, the extreme water temperature 

 occurred about twenty-four hours or more after the crest of the 



