^JMTLI<:S AND ABSTRACrS Ol'^ P A PIOUS 141 



the pans are made of on the rate of deposition, and the effect of setting the 

 pans above the level of the bottom. 



In the second category are included all factors that may have operated to 

 change the rate of deposition at various periods during post-Glacial time or 

 to remove material once deposited. Some of these are : 



1. Possible changes in climate of a large order that in the past may have 

 resulted in a greatly different rate of deposition of either silt or marl. 



2. Changes of climate in shorter cycles that may have caused pulsations in 

 the rate of deposition. In such case the danger would be that present-day 

 observations of the rate of deposition might be located in any part of the 

 cycle, and it would be impossible to tell from them what the true average rate 

 has been in the past. 



3. Changes in level of the lake may have influenced the rate of deposition. 

 Formerly it was larger than at present. Then it was gradually lowered to a 

 stage 4 or 5 feet below the present level, to which it has been raised by a dam. 

 It is believed, however, that these changes have not seriously affected the rate 

 of deposition of carbonates, but have only affected the rate of silt deposition. 



4. Great storms of rare occurrence may also have resulted in much more 

 rapid deposition, either of silt or marl, for short periods. 



5. Cultivation has exposed the surrounding area to wind and water erosion, 

 so that recent deposits may contain more silt than those in the past. Here, 

 again, it seems improbable that the rate of deposition of carbonates has been 

 materially changed. 



6. It is possible that leaching of carbonates was more rapid shortly after 

 the retreat of the glacier than it is at present, due to the larger quantities 

 then present at the surface of the drift. This may have resulted in more 

 rapid deposition. 



7. The last complication suggested is the effect of the large percentage of 

 decaying organic matter included with the deposit. What part this may play 

 is not evident at present, but must be carefully considered in interpreting the 

 results. 



Discussion 



Mr. Lancaster D. Burling : It will be of interest in this connection to record 

 that Mr. E. J. Whittaker, of the Geological Survey of Canada, is engaged in 

 the detailed study of MacKay Lake, in the vicinity of Ottawa, and that one of 

 his problems is the lapse of time since the last retreat of the sea from the 

 Saint Lawrence embayment. Lake Mendota is large and the lapse of time 

 depends on a complicated study of the original marl deposits in that portion 

 beyond the reach of sediment. McKay Lake is small and is surrounded by 

 marl outcrops which contribute a very thin annual layer of white, clastic marl. 

 These layers are separated from each other by a darker layer deposited each 

 spring, during the only period in which the lake receives outside drainage, and 

 the calculation of the lapse of time is here being made by actually counting 

 the number of layers preserved in the lake bottom. 



Prof. Frank Leverett called attention to an estimate of the length of post- 

 Wisconsin time, made by Dr. J. T. Scovell from the rate of filling of a lake in 

 northern Indiana, which was published some years ago by the Indiana Geo- 

 logical Survey. It is by combining several such estimates that we will reach 

 a proper understanding of the length of post-Glacial time. 



