1890.1 
451 
[Upham. 
those far northern latitudes enjoyed a temperate climate in the 
Miocene period ; and from the absence of glacial drift through the 
great series of Tertiary and Mesozoic formations we infer that 
climates as mild as those of the present day had prevailed during 
long eras before the ice age. With suddenness, geologically speak- 
ing, there came at the close of the Pliocene period a great refriger- 
ation of the climate of northern regions, overwhelming the Siberian 
herds of mammoths, and covering the surface of the northern half 
of North America and of northwestern Europe with snow and ice 
which increased to thousands of feet in depth. The conditions 
that seem requisite for the formation of these ice-sheets are long 
continued rather than an excessive cold and an abundant supply 
of moisture by storms, giving plentiful precipitation of snow dur- 
ing more of the year than now, so as to include in the time of snow 
accumulation not only the present winter but also the autumn andi 
spring months. The summers too were probably cooler than now,, 
for their heat was not sufficient to melt away the accumulated 
snow, which gradually increased in thickness from year to year,, 
its lower part being changed to ice. When large portions of con- 
tinents became thus ice-covered, the storms sweeping over them* 
would be so rapidly cooled that the greater part of their snowfall 
would take place upon the borders of the ice-sheet, within prob- 
ably fifty to two hundred miles from its margin ; but the snow- 
fall during the advance of the ice was probably in excess of 
the amount of evaporation and melting over the whole ice-cov- 
ered area. In New England and New York the average as- 
cent of the surface of the ice was twenty-five to thirty feet per 
mile for the first one hundred to two hundred miles from its 
boundary. Toward its centre the slope diminished, as on the in- 
terior ice of Greenland ; but the ice-sheet enveloping the north- 
eastern part of North America probably attained, as estimated 
by Professor Dana, a maximum thickness of about two miles on 
the Laurentian highlands between the river Saint Lawrence and 
Hudson bay. 
When the Quaternary ice-sheets occupied their greatest area, at 
the culmination of the effects of the extraordinary climatic condi- 
tions of this period, the southern border of the ice crossed the northern 
United States from Nantucket, Martha’s Vineyard, Long Island, 
and northern New Jersey, through Pennsylvania into southwestern 
New York, thence west-southwesterly to southern Illinois and 
