LHAr. III.] CONDITIONS AFFECTING DISTRIBUTION. 41 



great valleys on the sontliern side of the Alps pouied 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 wdiole 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 w^ater. 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 

 w^hich 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 evergTcen, to flourish Avithin twelv^e 

 degrees of the Pole. In the Pliocene period we find ourselves 



