642 THE POPULAR SCIENCE MONTHLY. 



in these arctic fetters — as illustrated by the Brazilian researches of the 

 lamented Agassiz. 



One of the latest and most extensive works relating to the Ice 

 period is given to the world by James Geikie, of the Royal Geologi- 

 cal Survey of Scotland. The subject is treated of with special refer- 

 ence to the phenomena exhibited in the north of Europe. He subdi- 

 vides the Ice age as follows : 



1. Preglacial Period. — This is seen best in the "Norwich crag," 

 where remains of the elephant and mastodon are found in peat-beds, 

 and these are indications of approaching cold. 



2. First Cycle of Cold. — This exhibits intense glacial conditions, 

 with great confluent glaciers ; intermediate mild and warm periods ; 

 arctic and southern mammalia visit Britain alternately, according as 

 climatal conditions become suited to their needs. This is followed by 

 an arctic climate with the mountains covered by snow and ice, the gla- 

 ciers ceasing to be confluent. The era terminates with local glaciers. 

 The deposits laid down are chiefly the " till " and " bowlder-clay," 

 with a few stratified sands. 



3. Last Interglacial Period. — In this Britain is at first insular, with 

 cold climate ; next continental, with climate changing from cold to 

 temperate and genial, and again to temperate. In early stages of the 

 continental condition, the arctic mammalia invade Britain. Subse- 

 quently these disappear, and are succeeded by the hippopotamus, etc. ; 

 afterward submergence ensues, and the islands are again insulated, 

 perhaps before the climate became again suited to arctic mammalia. 

 At the close of this period the land sinks, reaching the depth of 

 2,000 feet below its present level in Wales. The deposits of this 

 era are cave - accumulations, river - gravels, and high level beaches. 

 The human implements found with the extinct mammalia are of stone, 

 and of the rudest construction. 



4. Last Glacial Period. — This was a time of floating ice, compar- 

 able with the conditions imagined by the earlier writers for the whole 

 drift period. The climate was arctic, icebergs floated over most of 

 the land, enormous blocks of stone got stranded upon the hill-tops, 

 moraines clogged up valleys, and toward the termination of the pe- 

 riod local glaciers manifested the final effort of the ice to gain the 

 mastery. Remains of boreal shells and the mammoth occur for the 

 last time in the frozen sands and ancient beaches. 



5. Preglacial Period. — The land has regained its present level, ter- 

 races are formed by immense rivers, arctic forms of life have disap- 

 peared, and the era of bronze and iron implements shows what prog- 

 ress man has made in the arts. 



Till. — Throughout the length and breadth of Scotland occur scat- 

 tered heaps and ragged sheets of sand, gravel, and coarse debris, to- 

 gether with wide-spread deposits of clay largely obscuring the solid 

 ledges. In the Highland and upland districts these deposits seem to be 



