CONDITIONS OF PRESENT GLACIATION DISCUSSED. 321 



other favorable conditions must concur with the presence of cold, in order 

 that the snow-fall may be large in any particular region ; and, since cold 

 diminishes evaporation, it is plain that lowering the mean temperature of 

 the earth will have a powerful influence in hindering the occurrence of that 

 peculiar combination of circumstances which is needed in order to bring 

 about a large amount of precipitation, whether in the form of rain or snow. 

 As a proof of this — the necessary result of the climatic and topographical 

 conditions prevailing throughout the earth — it has been shown in the 

 present section that areas of excessive cold are abundanth' enough spread 

 out over the surface of the globe ; but that regions in which snow lies per- 

 manently on the ground, and becomes exposed to the conditions necessary 

 lor its transformation into ice, are very much less extensive. 



In the opinion of the present writer, we are justified in drawing, from the 

 facts here presented, the inference that we have no right to assume as 

 having existed during the Glacial epoch a period of intense cold, or even a 

 lower mean temperature than that now prevailing over the earth. Abundant 

 precipitation being demanded for the formation of extensive snow-fields and 

 large glaciers, and that abundance being dependent for its existence pri- 

 marily on a high temperature, it is evident that we can never have any such 

 thing as a general glacial period extending over the whole earth. If, on 

 examination of the facts, it should appear that certain regions were once 

 more or less heavUy glaciated, where now snow and ice no longer exist, or, 

 if existing, are present in much diminished quantity, we shall not be justified 

 in supposing that this condition of things can be accounted for by simply 

 lowering the mean temperature of the earth : on the contrary, a generally 

 higher temperature, being attended by more copious precipitation, is a priori 

 just as likely to be a favorable condition for producing the demonstrated 

 effects; while the local diminution of temperature necessary for causing 

 that precipitation to take the form of snow instead of water is to be sought 

 for in causes acting only over certain limited areas, and only in those local- 

 ities where reliable observations show that such effects actually did take 

 place. In short, the only general Glacial epoch which can possibly occur on 

 the earth will be that in which refrigeration has advanced so far that '' Polar 

 ice-caps " have been formed by the freezing of the ocean, at both poles. 

 From that point of view it is not impossible that we have already entered 

 upon the Glacial epoch, and it is not in the power of any one to say that 

 there is not now more snow and ice upon the earth than there has been at 



