"ME DIVISIONS OF THE ICE AGE. 4.09 
part of the Ice age, the general epeirogenic uplift of the 
continental area bridged the Strait of Gibraltar and the 
centre of the Mediterranean Sea from Tunis to Sicily and 
Italy, as shown by Dawkins and Geikie, raising the land in 
both regions about 2,000 feet higher than now, and affording 
a passage to the great African mammals. They also crossed 
where the shallow 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 north-eastern England. 
LENGTH oF PosTGLACIAL TIME. 
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 in- 
vestigation of the age of the peat beds in the Somme valley 
and of the alluvial deposits of the Tiniére, tributary to the 
Lake of Geneva, which latter had been differently inter- 
preted by Morlot; on the study of the recession of the Falls of 
St. Anthony, by Professor 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 dura- 
tion 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; and that it 
was completed, nearly as now about 5,000 years ago, with 
remnants of the old ice-sheets lingermg ever since in Alaska 
and Greenland and on the mountainous plateaus of Norway. 
The incursion of the Neolithic men and the doom of their 
Paleolithic predecessors belong thus probably 6,000 or 7,000 
years ago. 
LENGTH OF GLACIAL AND PALMOLITHIC ‘TIME. 
The duration of the Ice age cannot be so reliably deter- 
mined, but the researches of Geikie, Chamberlin, and many 
others convince us that-on both sides of the North Atlantic 
