140 PROCEEDINGS OF THE ALBANY MEETING 



Determination of Rate of Deposition 



It is proposed to place sediment pans in each of the locations where the 

 borings are made and collect the deposit which accumulates. One plan that 

 seems desirable is to make these pans with a permanent partition in the 

 middle, so that collections can be made at short intervals from one-half and 

 the sediment allowed to accumulate in the other half for a full year before it 

 is removed. This will tell whether or not part of the material is taken back 

 into solution at certain times in the year when the water becomes more 

 strongly acid. 



From such data as are at hand it appears that the deposit for each year 

 will be sufficient in amount to be measurable with accuracy; in fact, it was 

 surprising to find the amounts so large when the following quantitative esti- 

 mates were made. 



In experiments relating to bottom temperatures, carried on by Birge and 

 Juday, they drove pipes 7 or 8 feet without penetrating through the marl ; so 

 that we know there is a greater thickness of marl than this. Marl deposits 

 as thick as 30 feet are not uncommon. Therefore 3 meters and 8 meters may 

 be taken as reasonable limits of the thickness that exists in Lake Mendota. 

 If the duration of post-Glacial time be assumed to be between the limits of 

 8,000 and 80,000 years, the yearly deposit for the shorter time would be be- 

 tween 960 and 360 grams per square meter. For the 80,000-year period it 

 would be between 96 and 36 grams per square meter. The average thickness 

 of the annual deposit would be one-tenth millimeter if the deposit were 4 

 meters thick and post-Glacial time assumed at 40,000 years. These are merely 

 estimates to give some idea of the conditions of measurements that must be 

 met. 



If annual layers can be distinguished and counted, they will make a very 

 good check on the quantitative measurements. Doctor Birge states that cer- 

 tain minute Crustacea (Daphnia) deposit their winter eggs in cases (ephippia) 

 in great abundance, and so offer a possibility that careful study will make 

 possible the identification of annual layers. The careful quantitative study of 

 the plankton now being carried on by Birge and Juday for the State Survey 

 may also suggest other criteria of value. 



Factoks affecting Accuracy of Results 



These may be divided into two classes — the mechanical and the general. 

 Suggestions relating to the points mentioned, or others, will be most gratefully 

 received. 



In the first category are included : 



1. The difficulty of getting a complete core of a length possibly as great as 

 8 or 10 meters in depths of water var.^ing from 60 to 80 feet. 



2. The difficulty of handling and drying this core to preserve it in good 

 condition for study. 



3. The difficulty of getting the deposit in pans. Here must be considered 

 the stirring up of the bottom when the pans are set; the devising of some 

 satisfactory method of marking the location so that inquisitive fishermen and 

 boating parties will not disturb them; the possible influence of the material 



