CAVE ENVIRONMENT. 



THE NATURE OF THE CAVE ENVIRONMENT. 



Each cave is a distinct unit of environment and needs special consideration. 

 In the present work we can deal only with the general features of this environment. 

 The chief element for consideration is the absence or reduction of the amount of 

 light and the relative constancy of other physical conditions. On this basis a 

 cave may be divided into three regions: (i) the twilight region just within the 

 cave, bounded by the distance to which light penetrates from without this 

 part shades generally from epigean conditions to the real cave conditions; (2) the 

 region of fluctuating temperatures ; (3) the inner cave region . 



These different sections occupy greatly variable parts of different caves. In 

 Mammoth Cave the twilight region is large enough to contain a tennis court and 

 reaches some distance beyond the "iron door." Some Cuban caves are entirely of 

 the twilight character, usually containing an abundant fauna, consisting largely of 

 occasional, regular, or accidental visitors from the outside. The second region in 

 Mammoth Cave reaches to the Mammoth Dome. On a cold winter day I found 

 ice stalagmites on the floor of the entrance gallery just before it enters the dome. 

 In certain of the ice caves the entire portion beyond the twilight area may belong to 

 this section. In caves of the tropics, on the other hand, it may not exist at all. 

 The third part is the cave par excellence the inner section, but little influenced 

 by external conditions. Here there is absolute darkness at all times, both day and 

 night, summer and winter following each other without very decided change in 

 temperature. 



The temperature differs in the various parts of the same cave and also changes 

 slightly with the seasons. In the center of the Shawnee Cave at Mitchell the fluc- 

 tuations in temperature during a week do not equal the error of the recording ther- 

 mograph arising from unequal trimming of the paper, the absorption of water, 

 etc. The total fluctuation during a year is 2.2 C. It is remarkable that this 

 record of cave temperature is taken in a cave open at both ends with a current of 

 air flowing through it at times. The instrument is placed where it would be least 

 affected by these currents, that is, in a large room near the center of the cave about 

 15 feet above water-level. 



Glacieres, or ice caves, are found in various places. They exist wherever the 

 prevailing direction of the winds and nature of the cave causes a strong inflow of 

 air during the winter, reducing the temperature to below the freezing point. The 

 summer winds do not blow in the same direction, and convection currents are pre- 

 vented by the nature of the cave. 1 



Between June, 1906, and February, 1908, the fluctuations in the temperature in 

 the water where it leaves Shawnee Cave ranged from a maximum of 17.3 C. to 

 7.4, or through about 10 C. 2 



1 A very extensive list and excellent account of glacieres is given by Balch in his Glacieres or Freezing Caverns, 

 1900. Concerning the cause of glacieres, he says, on page 148: "The cold air of winter sinks into and permeates 

 the cave, and in course of time freezes up all the water which, in the shape of melting snow or cold winter rain 

 or spring water, finds its way in; and once ice is formed it remains long after ice in the surrounding open country 

 has melted away, because heat penetrates with difficulty into the cave." 



2 This range becomes interesting when compared with the range of temperatures in a lake. Professor Birge 

 gives the ranges of the water at the surface and at the depth of 18 m. for Lake Mendota: 



Surface, 1895 . . . o to 24 Bottom, 1895 i-5 t I 7- I 



Surface, 1896 . . . o to 26 Bottom, 1896 ... 2 to 16 



