Vol. 57.] CLIMATE OP THE PLEISTOCENE EPOCH. 417 



more perhaps than to oceanic currents, that the comparatively mild 

 winter climate of those countries is due, while a contrary influence is 

 exerted by the cold winds which prevail over the regions of Labrador 

 and a part of North America, situated on the west side of the 

 cyclonic disturbances of the North Atlantic and to the east of the 

 American anticyclone. 



The influence of areas of high and low pressure upon climate 

 depends, not merely on their relative position, but on their form 

 and alignment. The air set in motion by a cyclone, circular in 

 form, and of small diameter, does not travel far ; on the contrary, 

 a larger cyclone or one elliptical in shape, and so situated that it& 

 longest axis stretches a considerable distance from south to north, 

 causes, on the one side of its centre, the rapid transference of 

 volumes of heated air from regions far to the south, and, on the 

 other side, of chilled air from the frozen north (see, for example, 

 fig. 11, p. 437). Isobaric charts accordingly show that changes of 

 weather such as those characteristic of the British climate are not only 

 due to the daily or seasonal shifting or oscillation of the areas of low 

 and of high pressure, but also to the continual changes in their form. 

 In regions of disturbance like that of the North Atlantic, cyclones 

 and anticyclones are, during portions of the year, in constant 

 movement, pressing upon and eating into each other, and changing 

 in form like the waves of a choppy sea. A comparatively slight 

 disturbance may thus alter their shape, position, or alignment, and the 

 direction of the winds at any spot.'^ Any important atmospheric 

 disturbance at one point, however brought about, may consequently 

 make its influence felt at a considerable distance from the focus of 

 its origin. 



Keeping these well-known facts in mind, we may now enquire 

 how far the different meteorological conditions which may have been 

 set up during the existence of ice-sheets in North America and in 

 Eurasia, can be shown to be in harmony with, or explanatory of, the 

 teachings of geology. These conditions were probably of a more 

 permanent character than those now obtaining, since there would 

 not have been then so great a difference as that which now exists 

 between the winter and summer temperatures and barometric 

 pressures of the continental regions. Anticyclonic centres may have 

 existed at all seasons over the ice-clad areas,' while over the land 

 to the south of them, and over the ocean, regions of comparative 

 warmth, low-pressure conditions may generally have prevailed. 

 The stronger contrast between the cyclonic and anticyclonic systems 



^ The interdependence of the movements of the high- and low-pressure 

 systems was shown in an interesting paper by Major H. E. Rawson, Quart. Journ. 

 Eoy. Met. See. vol. xxiv (1898) p. 180. 



- The meteorological observations made by the British Antarctic Expedition 

 in lat. 71° S., showing a great preponderance of southerly over northerly winds, 

 both in winter and summer, seem to favour the theory that the ice-sheet of the 

 Antarctic regions is prevalently anticyclonic. See Borchgrevink's ' First on the 

 Antarctic Continent ' 1901, p. 304. 



