250 
COMPTE-RENDU. QUATRIEME PARTIE 
plains in tlie principal river valleys draining’ away Irom tlie de- 
parüng icc, these beds werc deeply croded by tbc streams as 
soon as Uie ice-front bad so far receded ihat tlie supplies of 
water and drifl froin ils nielling ceascd. Mucb of llie valley 
drift was soon removed by tlie river channclling, and its rem- 
ua nts, being- left as terraces on the sides of the valleys, bave 
caused tliis lirst stage of tlie Postglacial period to be named by 
Dana the Terrace epoch. In less vigorous action tlie streams 
bave conlinned at tbe saine work to tbc présent day, so tbat 
tbis term niay be exlcnded aiso to comprise this wbole period. 
In varions localities wc are able to nieasure tlie présent rate 
of érosion of gorges below -walerfalls, and tbe lengtb of tbe post- 
glacial gorge divided by tbe rate of recession of tbe falls gives 
approximately tbe time since tbe Ice âge. Sucb measurenients 
of tbe gorge and Falls of St. Antbony by Prof. N. H. Wincbell 
sbow tbe lengtb of tbe Postglacial or recent period in Minnesota 
to bave been about 8000 years ; and from tbe surveys of Nia- 
gara Falls, Mr. G. K. Gilbert estimated it to bave been 7000 years, 
more or less. From tbe rates of wave-ciitling along tbe sides 
of lake Michigan and the conséquent accumulation of sand around 
the South end of tbe lake, Dr. E. Andrews believes tbat tbe land 
lhere became uncovered from its ice-sbeet not more tban 
7500 years ago. Prof. G. Frederick Wright obtains a similai’ 
resiill from the rate of filling of kettle-holcs among tbe gravel 
knolls and ridges callcd kames and cskers, and likewise from tbe 
érosion of valleys by streams tribiitary to lake Erie ; and Prob 
Ben. K. Emerson, from the rate of deqiosition of modified drift 
in tbe Connecticut valley at Northampton (Mass.), tbinks tbat 
the time since tbe Glacial period cannot exceed 10 000 years. An 
eqnally small estimatc is also indicaled by the sludies of Gilbert 
and Russell for the time since the last great rise of the Pleisto- 
cene lakes Bonneville and Labontan, lying in Utah and Nevada, 
Avithin the arid Great Basin of interior drainage, wbicb are be- 
lievcd to bave been contemporaneous witb the great extension 
of ice-sheets upon the northern part of tbc North American con- 
tinent. 
Prof. James Geikie maintains tbat tbe use of paleolitbic impD' 
monts bad ceased, and tbat early man in Europe niade neolithir 
(polisbed) implemcnts, before tbe recession of the ice-sbeet froin 
Scolland, Denmark, and tbe Scandinavian peninsula ; and Prest- 
wich suggests tbat the dawn of civilization in Egypt, China and 
