Yol. 57.] THE CLITJATE OF THE PLEISTOCEXE EPOCH. 4ol 



in the physical conditions of the Glacial Period must have been 

 accompanied by disturbances of the meteorological equilibrium. 



As before stated, low-pressure systems "VYould have been prevalent 

 during the winter over the North Pacific and the North Atlantic, 

 but probably with a somewhat different statistical alignment from 

 that of the present day. 



The form of cyclones corresponds more or less closely with that of 

 the anticyclones adjoining them. Isobaric lines arrange themselves 

 around centres of high pressure on the one hand and of low pressure 

 on the other, but they represent portions of one system of atmo- 

 spheric circulation, and must, to a great extent, be roughly parallel 

 one to the other. Hence, if we can ascertain the position and 

 form of the anticyclones at any period, we may form a general 

 idea as to the position and alignment of the cyclones complementary 

 to them, the alignment of the latter being for our present purpose 

 specially important. 



The statistical alignment of the North Atlantic cyclone in winter 

 during the maximum giaciation of America (fig. IS, p. 452) may 

 therefore have been of a character similar to that of our own 

 era at the same season, its north-western margin being more or 

 less parallel to that of the American ice-sheet. It may have been 

 situated, however, farther south, because the North American 

 anticyclone would then have been north and north-west of it. 

 South-easterly winds would have been prevalent on the western 

 side of the latter, preventing, together with the cyclonic winds 

 already referred to, the permanent accumulation of ice to the west 

 of the Mississippi and in Alaska. 



The North Pacific cyclone, wit,h its longest axis parallel to the 

 western edge of the American ice, may have then pointed statisti- 

 cally south-east and north-west, instead of east and west as at 

 present, carrying warm south-easterly winds into that part of 

 the Polar basin which lies west of Behring Strait, and over the 

 northern coasts of Siberia ; and such a state of things may have 

 continued until a late stage of the Pleistocene Epoch. This view 

 offers an explanation, as we have seen (§ III, p. 422), of the 

 former abundance of mammalian life in that region. 



While, therefore, it is not difficult to understand that the 

 winter-distribution of pressure in the Atlantic during the maximum 

 giaciation of North America may have been of a similar character 

 generally to that of the present day, causing severe seasons in 

 that continent to coincide with milder weather in Europe,^ it is not 

 so easy to ascertain the baric conditions which may have existed 

 during the summer at the same period. The daily synoptic charts 

 afford us no assistance, since the arrangement of the isobars, 

 which must have then prevailed at that season, is never met with 



•^ My hypothetical chart (fig. 18, p. 452), showing the possible distribution 

 of pressure in winter during the maximum giaciation of North America, cor- 

 responds closely, as to its general principles, with the isobaric chart for January 

 of the present era (fig. 17, p. 450). 



