610 THE ICE AGE IN NORTH AMEBIC A. 



In searching for an explanation of the former increase in 

 size of these bodies of water, the conditions of the Glacial 

 period naturally present themselves as furnishing an ade- 

 quate cause. Glaciers, however, never occupied much of the 

 territory, being found only to a limited extent in the border- 

 ing mountain-ranges. But the proximity of the glaciated 

 region, and, indeed, the general conditions favoring the pro- 

 duction of the Glacial period in North America, would be 

 ample to produce the temporary enlargement of these lakes. 

 A slight increase in precipitation, or a slight diminution of 

 temperature, would either of them cause a rise in the water 

 until the balance should be readjusted between the rainfall 

 and the evaporation. 



It would seem that there is here also a significant record 

 of an interglacial epoch, for the lakes have had two periods 

 of increase, with an arid period intervening. During the 

 first rise of the lakes, sediment to the extent of one hundred 

 and fifty feet in thickness was deposited. There was then a 

 dry period, in which the lakes were reduced to their present 

 dimensions, or even smaller, when these first deposits were 

 subjected to a period of erosion by surface streams, and partly 

 covered with gravel. There was also upon it a deposit of 

 great quantities of compact stony tufa precipitated from 

 waters saturated with calcium carbonate. After the period 

 of low water there was a subsequent reflooding of the basin, 

 which reached a horizon thirty feet higher than the first. 

 During this rise a deposit of thinolite took place, and of other 

 substances whose position and character serve to note the 

 changes. Subsequent to this rise the evaporation proceeded 

 at an increased rate until the basins were completely desic- 

 cated, and only began to refill within a period which Mr. 

 Eussell estimates to be less than three hundred years. All 

 this, however, might have occurred within the space of a few 

 thousand years, and does not, independently of other evidence, 

 go far to establish the complete duality of the Ice age. 



As to the date of the expansion of these lakes, Mr. Rus- 

 sell expresses it as his opinion that "the last desiccation oc- 



