Primitive Man i?i the Somme Valley. — UpJiam. 361 
low strait of Dover now is, entering Britain; and during the 
oncoming and culmination of the Ice age the British Isles 
were united with the continent by a very broad land surface, 
reaching far west of the Channel and occupying the basin of 
the North sea and nearly all the area between Scotland and 
Norway, until the Scandinavian ice-sheet covered that land 
plain to northeastern England. 
Our estimates of the duration of the Postglacial period are 
based on computations by Andrews from the shore erosion 
and beach sand accumulation of lake Michigan; on his inves- 
tigation, before cited, of the age of the peat beds in the Somme 
valley and of the alluvial fan deposits of the Tiniere, tributary 
to lake Geneva, which latter had been differently interpreted 
by. Morlot; on the study of the recession of the falls of St. 
Anthony by Prof. N. H. Winchell; on that of Niagara falls 
by Gilbert and others; on Dr. Robert Bell's observations of 
the extent of subaerial erosion of limestone rocks in Canada: 
and on many other careful studies in both North America 
and Europe. These estimates concur so well that the dur- 
ation which they give approximately as between 5,000 and 
10,000 years may be confidently accepted as the measure of 
Postglacial time. It may be otherwise and better stated that 
the departure of the ice-sheets on these continents probably 
occupied some 5,000 years, known as the Champlain epoch 
from marine deposits overlying the glacial drift in the basin of 
lake Champlain; and that it was completed, nearly as now. 
about 5,000 years ago, with remnants of the old ice-sheets lin- 
gering ever since in Alaska and Greenland and on the moun- 
tainous plateaus of Norway. The incursion of the Neolithic 
men and doom of their Palaeolithic predecessors belong thus 
probably 6,000 or 7,000 years ago. 
The duration of the Ice age cannot be so reliably deter- 
mined; but the researches of Geikie, Chamberlin, and manv 
others, convince us that on both sides of the North Atlantic 
glaciation was approximately synchronous, with parallelism in 
its beginning, fluctuations, and end, through a period ten to 
twenty times as long as that which has followed it, or of some- 
what such ratio. In other words, the Glacial period, from the 
time of the Palaeolithic men at Menchecourt and St. Acheul 
to the Neolithic immigration and the not widely later re- 
