ICE AGES — SIMPSON 299 



owing to increased temperature at the maxima of the radiation. The 

 ice which accumulates year by year in the specified area must escape; 

 if the area is a mountain top, the escape will be by way of glaciers; 

 if it is a polar cap, the escape will be by an ice sheet spreading over 

 the surrounding country. In either case the ice is transported into 

 regions where the melting is greater than the snowfall, and the greater 

 the annual accumulation the farther will the ice penetrate, down the 

 mountain side if a glacier and into lower latitudes if an ice sheet. 

 Curve V shows the advance and retreat of the ends of the glaciers 

 or of the front of an ice sheet. 



Examining the last curve, we see a close resemblance to Penck and 

 Bruckner's curve for the height of the snow line in the Alps (fig. 2). 

 In each there are four advances of the ice. The advances are ar- 

 ranged in pairs, with short interglacials between the advances in the 

 pairs, and the pairs separated by a longer interglacial. This arrange- 

 ment comes from the fact that there are two glacial epochs for each 

 maximum of the solar radiation. The interglacials have different 

 climates according as they occur at the maximum or a minimum of 

 the solar radiation. The former are warm and wet and the latter are 

 cold and dry. We are at present approaching a minimum of solar 

 radiation. In consequence our rivers are smaller and our tempera- 

 ture lower than they were in the last interglacial. We now see why 

 there were only two pluvial periods in nonglaciated regions for the 

 four glacial epochs in polar regions and on mountains. If the theory 

 is correct, the last interglacial — the Riss-Wiirm interglacial — was a 

 warm one, and to it the evidence for a warm wet climate which I 

 have described above should belong. 



Thus we see how two oscillations of radiation have produced in 

 nonglaciated regions two pluvial periods, and in glaciated regions four 

 glacial epochs separated by three interglacial epochs, two of which 

 were warm and wet and one cold and dry. If the theory is correct 

 we are now living in a cold dry epoch owing to the decrease of solar 

 radiation from its last maximum. If the solar radiation again in- 

 creases, there will be another glacial epoch and our epoch will become 

 a second cold dry interglacial. 



An obvious objection to the explanation of the last Ice Age which 

 I have given above is that with their present temperatures no amount 

 of precipitation would result in the formation of an ice sheet over 

 England and Scotland and still less over Ireland; yet we know that 

 such ice sheets did exist. The objection is still stronger when it is 

 remembered that the theory requires the increase ofprecipita tion to 

 be accompanied by an increase of temperature. 



There is another problem which has puzzled everyone who has 

 studied the cause of the Ice Ages. If we plot on a circumpolar map 



