402 



its outlet may become obstructed. The result is the formation of a pond. 

 If fi young sinkhole is obstructed, a small and relatively deep pond re- 

 sults. The obstruction of an old sinkhole results in the formation of a 

 shallow pond of considerable area. 



Destruction. — The ponds art' uo sooner formed than their destruction 

 begins by means of those factors which destroy all such topographic forms. 

 Few of them overflow, anl these only for a short time. Plant deposition 

 and the deposition of silt are the two principal factors operating for their 

 destruction. A pond formed in a young sinkhole which is located at or 

 near the summit of a hill, i. e., near the level of an old peneplain, does 

 not have as much silt washed into it as does a pond formed in an older sink- 

 hole or one that is located on the lower slope of a hill. Plants are rela- 

 tively a much greater factor in the destruction of the former than in the 

 latter. 



Our pond belongs to the first class. It has some clay deposited in it, 

 but plant debris forms the major part of its sediment. The rate of its de- 

 struction is known approximately for a period of 24 years. In 1887, it was 

 about eight or nine feet in depth ("deep enough to swim a horse"). It is 

 now slightly less than four feet, a difference of four feet, or one-fifth foot 

 deposition per year. So far as I know, this is the only case where the rate 

 of plant deposition is reducible to CA^eu approximate figures. 



The water is usually clear. A scum of iron oxide was found on the 

 surface April 1, 1910. On August 12, 1910, the water had a dark purplish 

 tinge, due to the decay of organic matter. The only time the pond was seen 

 to be muddy was after the rain of July 14. On this date it was quite 

 opaque and of a yellowish tinge, from the suspended silt. Silt is carried 

 into the pond only after very heavy rains, for reasons previously stated. 



METHODS. 



For collecting insects, insect larvte, algae, amphibian larvae, etc., ordi- 

 nary insect nets and dip nets made of bobbinet and scrim, were used. A 

 very useful net for collecting micro-organisms, when quantitative work is 

 not demanded, is a sampling net, manufactured by the Simplex Net Com- 

 pany, Ithaca, New York. It is made of bolting cloth No. 20, is three inches 

 in diameter, twelve inches long, and is operated by being thrown out into 

 the water and then drawn in. The ring is quite heavy so that it will sink 



