102 DISCUSSIONS IN CLIMATOLOGY. 



cannot be thus changed till after the ice accumulation has 

 been considerably affected by other causes ,"-* 



There are some further reasons assigned, which will 

 be considered as we proceed. 



From what has already been shown, it will be seen 

 that the causes which led to the glacial epoch may be 

 classed under three distinct groups : — (1) the astro- 

 nomical, (2) the physical, and (3) the geographical. 

 This threefold division is distinctly recognised by Mr. 

 Wallace in the above quotations, as well as in all his 

 reasoning on the subject of geological climate. 



In the astronomical group the main elements are 

 the two following : — 1st, A high state of eccentricity 

 producing, on the hemisphere whose winter solstice 

 happens to be in aphelion, a long and cold winter with 

 a short and hot summer, and on the other hemisphere, 

 whose winter solstice, of course, at the time is in peri- 

 helion, a short and mild winter with a long and cool 

 summer ; 2nd, Precession, transferring these conditions 

 from the one hemisphere to the other alternately every 

 10,000 or 12,000 years. The physical elements are, of 

 course, the influence of snow and ice, ocean-currents, 

 aqueous vapour, clouds, fogs, and a host of other 

 things which have already been discussed at length 

 in Chapters II. and III.; while the geographical 

 consist of the particular distribution of land and 

 water, elevations or depressions in the sea-bottom, 

 contour of the sea coast, and other geographical con- 

 ditions influencing the flow of ocean-currents. 



It is to the influence of physical agencies, however, 

 that the glacial epoch is more directly due. The main 

 function of the astronomical agents is to set and keep 

 the physical agencies in operation, and also to deter- 



* " Island Life," p. 148. 



