602 WOODWARD, POLAR QLACIATION, ETC. 



evaporation, but much of the snow which would doubtless reach 

 the Polar regions is intercepted by mountain-chains and added to 

 local glacier-systems, such as the Alps, the Himalayas, and other 

 high northern ranges. (Compare Diagrams, Figs. 8 and 9.) 



That the difference is due to lessened precipitation, and not to 

 differences of temperature, will be seen by following the isotherm 

 of 30° around each hemisphere. 



In the southern hemisphere it deviates but little from the line 

 of 60° S. lat. In the north it is much more irregular, but the 

 mean is again about 60° N. lat., proving that if the precipitation 

 were the same, tliere would be as much snow and ice north of lat. 

 60° N. as there is south of lat. 60° S. 



{b.) Accumulations of snow have only been observed on one 

 planet viz., Mars, which, with an obliquity of 31^°, is glaciated at 

 both Poles at the same time. 



Judging, then, from analogy, we might expect the Glacial 

 Period to have been contemporaneous in both hemispheres. 



(c.) Many plants and animals are found in both the northern 

 and southern Temperate Zones, having close affinities, and even 

 pointing to a common origin, yet separated by the whole width of 

 the Tropics, which they cannot now pass. 



Mr. Darwin has explained their presence by supposing that 

 during the Glacial Period they were driven to the high lands of 

 the Tropics by the advancing ice, and that on its retreat they 

 followed it north and south. 



A glacial period in one hemisphere only would not, Mr. Belt 

 thinks, afford this means of migration. The plants and animals 

 driven south by the northern ice would always have a hot zone to 

 the south of them which they could not pass. 



{d.) Mr. Alfked Tylor has suggested that the piling up of ice 

 in the nothern hemispheres during the Glacial Period Avould lower 

 the level of the general ocean 600 feet. 



Mr. Croll, on the contrary, in his recent papers on the subject 

 {Geol. Mag., July and August, 1874), shows that, if each hemis- 

 phere were glaciated alternately, the level of the ocean would be 

 raised and 7iot lowered in the one in which the ice accumulated 

 (as already explained) by the melting of the ice at the opposite 

 pole, and the shifting of the centre of the earth's gravity towards 

 that covered by an ice-cap. 



Mr. Belt concludes that one Polar ice-cap could not materially 

 lower the general ocean, if the opposite pole were freed from ice ; 

 but, on the contrary, if the Glacial Period in the two hemispheres 

 were contemporaneous, then the water abstracted from the sea and 

 frozen into ice at the Poles, together with that impounded in the 

 great lakes of North America, North Europe, and North Asia, by 

 the blockade of the northern drainage of the continents with ice, 

 must have lowered the general level of the ocean to a great 

 extent. 



From data gathered in Central America, and in Siberia, Mr. 

 Belt estimates the lowering of the sea-level by the accumulation 

 of ice at the Poles could not have been less than 2,000 feet, and 

 may have been much more. 



