chap, nr.] CONDITIONS AFFECTING DISTRIBUTION. 
41 
great valleys on tlie southern side of the Alps poured down 
streams of ice which stretched far out into the plains of North- 
ern Italy, and have left their debris in the form of huge 
mountainous moraines, in some cases more than a thousand feet 
high. In Canada and New Hampshire the marks of moving ice 
are found on the tops of mountains from 3,000 to 5,000 feet 
high ; and the whole surface of the country around and to the 
north of the great lakes is scored by glaciers. Wherever the 
land was submerged during a part of this cold period, a deposit 
called boulder-clay, or glacial-drift has been formed. This is a 
mass of sand, clay, or gravel, full of angular or rounded stones 
of all sizes, up to huge blocks as large as a cottage ; and especi- 
ally characterized by these stones being distributed confusedly 
through it, the largest being as often near the top as near the 
bottom, and never sorted into layers of different sizes as in 
materials carried by water. Such deposits are known to be 
formed by glaciers and icebergs ; when deposited on the land by 
glaciers they form moraines, when carried into water and thus 
spread with more regularity over a wider area they form drift. 
This drift is rarely found except where there is other evidence of 
ice-action, and never south of the 40th parallel of latitude, to 
which in the northern hemisphere signs of ice-action extend. 
In the southern hemisphere, in Patagonia and in New Zealand, 
exactly similar phenomena occur. 
A very interesting confirmation of the reality of this cold 
epoch is derived from the study of fossil remains. Both the 
plants and animals of the Miocene period indicate that the 
climate of Central Europe was decidedly warmer or more equa- 
ble than it is now ; since the flora closely resembled that of the 
Southern United States, with a likeness also to that of Eastern 
Asia and Australia. Many of the shells were of tropical genera ; 
and there were numbers of large mammalia allied to the 
elephant, rhinoceros, and tapir. At the same time, or perhaps 
somewhat earlier, a temperate climate extended into the arctic 
regions, and allowed a magnificent vegetation of shrubs and 
forest trees, some of them evergreen, to flourish within twelve 
degrees of the Pole. In the Pliocene period we find ourselves 
