542 



CENOZOIC TIME. 



and continental Siberia on the east. Great Britain was probably at 

 the same time joined to Europe (p. 572), and to the islands on the 

 north. Scotland, as its fiords and the channels between the Hebrides 

 show, must have been at least 1,000 feet above its present level. 



2. Source of the Cold The subject of the origin of change in 



climate is treated of on page 714, where some of the sources of cold as 

 well as warm climates are explained. It is important to remember, in 

 this connection, that cold which covers a couutry with ice has in the 

 ice a source of perpetuation, since, whatever the heat of the sun, no 

 temperature is radiated from the surface higher than that of freezing, 

 or 32° F. 



If the cause of the Glacial cold was connected with a closing of the 

 Arctic regions against the tropical currents of the Atlantic (p. 713), 

 the North Atlantic ocean would have had greater warmth than now, 

 and this would have produced unusual evaporation, and hence unusual 

 precipitation on its cold borders. 



The theory that makes the era of cold dependent on the earth's 

 being in one of its periods of maximum eccentricity (p. 715), requires 

 that the Glacial era of the southern and northern hemispheres come at 

 different times 11,500 years apart. A source of evidence as to whether 

 there was such an interval or not, exists in the distribution of Quater- 

 nary animal and vegetable life ; but no facts have yet been observed 

 settling the question. 



3. Exterminations and migrations consequent on the approach of 

 the cold period. — The approach of the cold Glacial era probably pro- 

 duced that extermination of species which closed the Tertiary age, be- 

 sides causing the migration to more southern latitudes of species not 

 exterminated. Some facts illustrating the latter point are mentioned 

 on pages 532, 533. The former hardly needs illustration. The cold 

 must have come on with extremely slow progress. The extermina- 

 tion of the terrestial Tertiary mammals, or such as did not find shelter 

 to the South, may have been an early effect of the progressing refrig- 

 eration ; and, long before the glacier had reached its limits, species 

 adapted to a more rigorous climate, that is, those of Quaternary times, 

 may have begun to occupy the country. 



The Glacial period, which is here shown to have probably been an 

 era of high latitude elevation, was followed by one of unquestioned 

 depression — the Champlain period ; and to this period of depression 

 is here referred the closing part of the era of ice, that is, the period of 

 the melting or disappearance of the ice. 



