192 DISCUSSION OF THE DESICCATION QUESTION. 



of ice than there is at the present time. Hence the desiccation which has 

 tliere gone on for so long, and which is still proceeding, can in no wise be 

 connected witli the melting of any glacier, or set of glaciei-s, or great gen- 

 eral ice-sheet. It must be a phenomenon quite independent of the occur- 

 rence of the Glacial epoch. The same being, as already shown, true for a 

 large part of North America, and, as far iis the limited facts at our disposal 

 allow of an opinion being formed, for Africa and South America, we are 

 manifestly led to prosecute our inquiries in another direction, leaving the 

 consideration of tlie Glacial epoch to one side for the present, as not leading 

 us toward the solution of the problem before us. 



Only one aspect of the prevailing theory in regard to the melting of the 

 " great glacier " will be touched upon before proceeding to another branch 

 of our inquiry. It is this: it is assumed that the time of the melting of 

 large glaciers, and especially of an ice-sheet extending over a large area 

 of country, must necessarily have been one of great floods. This view can- 

 not, however, be accepted as being, -pruna facie, correct ; certainly it needs 

 some explanation and limitation. The question comes up, in the first place, 

 Avhether a mass of ice may not disappear by evaporation, so that its melting 

 away maj' be attended by no deluge of water. The vast quantities of snow 

 piled up in the Sierra Nevada during the winter, as a rule, do disappear 

 without producing great freshets. Such events as that of the flooding of the 

 Sacramento Valley in 1861-62 do not proceed from any ordinary melting 

 of the winter's snow under the influence of a sunnner sun ; they are pro- 

 duced by a deluge of warm rain suddenly precipitated upon a body of snow 

 ■which has just fallen in the Lower Sierra, and which has not had time to 

 become carried away in the ordinary manner by evajioration. Such very 

 sudden changes in tlie tenqierature of the air, or rather in the direction of 

 the air-currents, can be understood as a phenomenon of local occurrence ; 

 but it would not be easy to imagine a large part of a continent as being 

 subjected instantaneously, as it Avere, to such a climatic change. 



As an illustration of what is here meant to be stated, attention may be 

 called to the fact to wdiich much fuller reference will be made in the next 

 chapter, namely, that the glaciers of the Pyrenees, the Alps, and the Cau- 

 casus have, for the last thirty or forty jears, been in process of diminution ; 

 they have in feet been disappearing .so rapidly that, should this shrinking 

 be continued at the present rate for one or two hundred years longer, there 

 will hardly be any ice left on the Alpine summits. It does not apj^ear. 



