986 The American Naturalist. [December, 
receded that the supplies of water and drift from its melting 
ceased. Much of the valley drift was soon removed by the 
river channelling, and its remnants, being left as terraces on 
the sides of the valleys, caused this first stage of the Post- 
glacial period to be long ago named by Dana the Terrace 
epoch. In less vigorous action the streams have continued at 
the same work to the present day, so that this term may be 
extended also to comprise this whole period. 
In various localities we are able to measure the present 
rate of erosion of gorges below waterfalls, and the length of the 
postglacial gorge divided by the rate of recession of the falls 
gives approximately the time since the Ice age. Such meas- 
urements of the gorge and falls of St. Anthony by Professor 
N. H. Winchell, show the length of the Postglacial or Recent 
period in Minnesota to have been about 8000 years; and from 
the surveys of Niagara Falls, Mr. G. K. Gilbert estimated it to 
have been 7000 years, more or less. From the rates of wave- 
cutting along the sides of Lake Michigan and the consequent 
accumulation of sand around the south end of the lake, Dr. 
E. Andrews believes that the land there became uncovered 
from its ice-sheet not more than 7,500 years ago. Professor G. 
Frederick Wright obtains a similar result from the rate of 
filling of kettle-holes among the gravel knolls and ridges 
called kames and eskers, and likewise from the erosion of 
valleys by streams tributary to Lake Erie; and Professor Ben. 
K. Emerson, from the rate of deposition of modified drift in 
the Connecticut Valley at Northampton, Mass., thinks that the 
time since the Glacial period cannot exceed 10,000 years. An 
equally small estimate is also indicated by the studies of Gil- 
bert and Russell for the time since the last great rise of the 
Pleistocene lakes!Bonneville and Lahontan, lying in Utah and 
Nevada, within the arid Great Basin of interior drainage, 
which are believed to have been contemporaneous with the 
great extension of ice-sheets upon the northern part of the 
North American continent. 
Professor James Geikie maintains that the use of paleolithic 
implements had ceased, and that early man in Europe made 
neolithie (polished) implements, before the recession of the 
