470 MR. F. W. HARM.ER ON THE [^^o' IpOI* 



are usually of a more or less opposite character to those of thu 

 neighbouring land-tracts. 



During the Glacial Period, the regions covered by ice might have 

 been, to a greater or less extent, anticyclonic at all seasons, low- 

 pressure systems prevailing at the same time over the warmer 

 regions immediately to the south of them and over the adjoining 

 oceans. The relative positions of areas of high and low barometric 

 pressure, the direction of the prevalent winds, and the consequent 

 distribution of climatic zones, would in such a case have differed 

 from those of the present time : oceanic winds, with copious rain- 

 fall, may have prevailed over regions now arid, and mild winters 

 where they are now excessively severe. 



The teachings of geology will thus throw light on the meteoro- 

 logy of the past, and meteorology may explain the causes of former 

 cases of anomalous climate. 



At present, for example, dead shells are but seldom found on the 

 eastern shores of Norfolk and Suffolk, though they are constantly 

 driven on to the Dutch coast by westerly gales. The extraordinary 

 profusion of such debris in the Upper Crag-beds of East Anglia, 

 the littoral deposits of the North Sea in Pliocene times, suggests 

 that easterly gales were more common there at that period than 

 they are now. 



The prevalence of strong westerly winds in that region at present 

 is due to the fact that the centres of cyclonic storms approaching 

 Great Britain from the Atlantic, pass to the north or north-west. 

 When an anticyclone exists to the north, which is not often the 

 case during the winter, cyclones take a more southerly course, and 

 easterly gales are experienced in the Crag district. Such a state 

 of things existed, not improbably, in the later Pliocene Epoch, as 

 glacial conditions may have by that time established themselves, to a 

 greater extent than at present, upon the Scandinavian highlands. 



During the existence of anticyclonic conditions over the European 

 ice-sheet at the period of its maximum extension, when lower pres- 

 sures prevailed in the warmer areas south of it, cyclonic storms 

 may have passed farther south than they do at present, bringing 

 oceanic winds over the Saharan desert, which, it is known, formerly 

 enjoyed a more humid climate. 



The abundance of the mammoth in Pleistocene times along the 

 shores of the Polar Sea (where no trees can grow at present, owing 

 to the excessive severity of its winter-climate), may have occurred 

 during the existence of an ice-sheet in North America, when a 

 different statistical alignment of the Behring Strait cyclone, due 

 to the more northerly position of the American anticyclone at that 

 period, brought mild south-easterly winds from the Pacific over 

 Northern Siberia, ameliorating its winter -climate, just as the 

 prevalent alignment of the Icelandic cyclone now carries mild 

 south-westerly winds over Great Britain and Scandinavia, and 

 thence into the Polar regions at that season. 



The alternate humidity and desiccation, during the Pleistocene 

 Epoch, of the now arid basin of Nevada, where great lakes formerly 



