June, 1919] 



Fauna of Rock Bottom Ponds 



465 



of the ponds there was an original layer of quarry dust which 

 was from one-eighth to one-fourth inch thick. It is therefore 

 safe to assume that the amount of organic material from the 

 pond itself and of dust or other material from without which 

 had accumulated on the bottom of the one year and five year 

 ponds is negligible. A thick mat of algae in the one year pond 

 was furnishing a good source for such accumulations. The 

 great amount of plankton in this pond during its second summer 

 would also add to it. 



TABLE 23. 



'Temperature in parenthesis was taken at the bottom. 



The rich plankton of the ten year pond must form a consid- 

 erable layer of sediment each year. Added to this is undoubt- 

 edly quite an amount of material from the surrounding quarry 

 the floor of which was quite dusty. The twelve inches of material 

 recorded for the fifteen year pond was along one end where soil 

 from above had slipped over the edges. Thick mats of algce 

 were evidently the chief source of organic deposit. In the 

 thirty year pond the algal deposits were augmented by leaves 

 from surrounding trees. The normal accumulation of this 

 pond was increased by material washed down from the two 

 ponds above it. 



This series also furnish some data with regard to the rate 

 at which rooted vegetation can become established on a sub- 

 stratum originally of rock. The five year pond had reached 

 the point where it could support such vegetation in the form of 



