Dr. Croll's Hypotheses on Geological Climates. 235 



ciable difference in winter, and, as in the year there is no 

 difference in the quantity of heat received by the waters, I 

 think there will be no difference in the temperature of the 

 waters, and thus no influence of great excentricity and 

 winter in aphelion on the ocean temperatures, and also no 

 greater snowfall than now. As to the continents, I admit 

 that, though we are unable to calculate the rate of decrease of 

 temperature of the winter months under these conditions, there 

 is no doubt that it ivill be appreciable, and be the greater the less 

 a given place is under the influence of the seas. 



But what has this to do with glaciation ? Even now, the 

 temperatures in the interior of large continents are low enough 

 in midwinter to allow of the snow remaining on the ground for 

 some weeks, not only under 45° N., but under 40° N. And yet 

 we have no glaciers on the North- American continent, which 

 reaches to 71° N., or on the Asiatic, which reaches to 78° N., 

 except in high mountain-regions, because the snowfall of 

 winter is so small that it is melted in summer. Even the 

 mountains of N.E. Siberia have no glaciers. 



The greater part of the snow which lies on the ground inN.E. 

 Siberia falls in autumn, when the air contains vapour of water 

 enough to allow of a great precipitation ; the snowfall of 

 winter contributes very little. Now what would a further 

 lowering of the temperature of winter produce ? A further 

 diminution of the quantity of the falling snow. It would 

 then even sooner be melted in the warmer summer months. 



The cold of winter in the interior of large continents of 

 high latitudes, especially that of Asia, has very important in- 

 direct results : the high pressure and the resulting cold and 

 dry winds of winter, especially towards the S. and E. of the 

 region of high pressure. These winds, the cold dry winter 

 monsoon winds of Eastern Asia, are unfavourable to snowfall, 

 so that in the interior of Transbaikalia, for example, with mean 

 winter temperatures of —13° F. and below, there is generally 

 too little snow for sleighing ; and if the quantity of snow 

 falling near the mouth of the Amoor is larger, it falls almost 

 entirely in October and November, i. e. at the beginning of 

 the cold season, and in the few days with east winds, which 

 bring warmer and moister air from the seas, not yet frozen 

 in these months. 



A lower winter temperature and an earlier beginning of 

 the cold in the interior of Asia would increase the pressure 

 towards the north and the interior of the continent, and thus 

 give a greater impetus, strength, and duration to the dry N. W. 

 winds, and so be even less favourable to snowfall and an 

 accumulation of snow. 



