168 PROGRESS OF SCIENCE. 



noticed the accumulation of deposits in the valleys, due ta 

 their action ; Agassiz, following up this study, detected 

 other phenomena which revealed the nature of the climate 

 of the earth during a long series of ages which are now 

 called GLACIAL PERIOD. He discovered that the whole 

 of the Northern American and European continents had 

 been once covered with a field of ice, and therefore with 

 immense glaciers in every valley, which extended as far as 

 the south of Germany and France, and in America as far 

 south as the Scth, 48th, and even 45th degree of latitude 

 the present glaciers being the remnants as yet unmelted 

 of the cap of ice. This cap of ice, as far as it reached, 

 extinguished vegetation and life in the Northern hemi- 

 sphere. It accounts for several important facts : (i) for the 

 presence of huge boulders or boulder- clay foreign to the 

 region in which they are found, for both boulders and mud 

 must have been brought where found by glaciers ; (2) for 

 the presence, too, of drift (glacial debris) in valleys of different 

 formation ; (3) it accounts also for the polished and grooved 

 surface of rocks (striae) ; and (4) finally it accounts for the 

 striking difference between the kind of Flora and Fauna 

 which existed over the same area before the Glacial age, 

 and those which exist now in the same habitat. Before 

 the Glacial era, tropical plants and animals flourished all 

 over England and countries of the same latitude; the intense 

 cold, frost, ice, killed these or drove them southward, and 

 their place was taken by Arctic flora and fauna mosses,, 

 lichens, firs ; reindeer and Polar bears. And when the ice- 

 fields retired, the plants and animals which invaded the free 

 area were different from those of the pre-Glacial epoch and 

 from those of the Glacial epoch. Subsequent researches 

 have revealed the same changes at the same period to have 

 occurred also in Asia. The causes of this great catastrophe 

 which overtook the earth are as yet unascertained. It was 

 possibly due to the greater inclination of the earth's axis ; 

 or to the periodic elongation of the earth's orbit and the 

 concurrent effects of precession; or again, to an alteration 

 in the shape of the continents which would have diverted 

 the warm sea-currents from their present and pre-Glacial 



