1894.] Quaternary Time Divisible in Three Periods. 987 
ice-sheet from Scotland, Denmark and the Scandinavian pen- 
insula; and Prestwich suggests that the dawn of civilization 
in Egypt, China and India may have been coeval with the 
glaciation of northwestern Europe. In Wales and Yorkshire 
the amount of denudation of limestone rocks on which drift 
boulders lie has been regarded by Mr. D. Mackintosh as proof 
that a period of not more than 6000 years has elapsed since 
the boulders were left in their positions. The vertical extent 
of this denudation, averaging about six inches, is nearly the 
same with that observed in the southwest part of the Province 
of Quebee by Sir William Logan and Dr. Robert Bell, where 
veins of quartz marked with glacial strie stand out to various 
heights not exceeding one foot above the weathered surface of 
the enclosing limestone. 
From this wide range of concurrent but independent testi- 
monies, we may accept it as practically demonstrated that the 
ice-sheets disappeared only 6000 to 10,000 years ago. Within 
this period are to be comprised the successive stages of man's 
development of the arts, from the time when his best imple- 
ments were made of polished stonethrough the ages of bronze, 
iron, and finally steel, to the present time when steel, steam 
and electricity seem to bring all nations into close alliance. 
EstiMATED DURATION OF THE QUATERNARY Ena. 
Arranged in chronologic order, we have derived for the 
three parts of the Quaternary era, as here defined, the follow- 
ing estimates of their duration: the Lafayette period or time 
of preglacial epeirogenie elevation, with the deposition and 
erosion of the Lafayette beds, some 60,000 to 120,000 years ; 
the Glacial period, regarded as continuous, without interglacial 
epochs, attending the culmination of the uplift, but terminat- 
ing after the subsidence of the glaciated region, 20,000 to 
30,000 years; and the Postglacial or Recent period, extending 
to the present time, 6000 to 10,000 years. In total, the Quater- 
nary era in North America, therefore, has comprised probably 
about 100,000 or 150,000 years, its latest third or fourth part 
being the Ice age and subsequent time. The Tertiary era 
appears by the changes of its mollusean faunas to have been 
