of Secular Changes of Climate. 95 



ill after the ice-accumulation has been considerably affected by 

 other causes." — P. 148. 



There are some further reasons assigned, which will be 

 considered as w T e 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 : — (l)the astronomical, (2) the physical, 

 and (3) the geographical. This threefold division is dis- 

 tinctly recognized 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 aphe- 

 lion, 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 perihelion, 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* : while the 

 geographical consist of the particular distribution of land 

 and water, elevations or depressions in the sea-bottom, con- 

 tour of the sea-coast, and other geographical conditions influ- 

 encing 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 determine the character of 

 their operations. For example, the position of the winter 

 solstice in relation to the aphelion or to the perihelion, during 

 a high state of eccentricity, determines whether the physical 

 agencies will produce on a given hemisphere a glacial or a 

 warm condition of climate ; while precession determines which 

 of the two hemispheres shall be the glaciated and which the 

 warm. In one respect we may say that the astronomical 

 causes produce glaciation by means of the physical agencies. 



The geographical conditions, however, cannot properly be 

 considered to be causes in the sense in which the astronomical 

 and physical are. They are more properly conditions to the 

 production of a glacial epoch than causes. They cannot be 



* ' Climate and Time/ chap, iv.j ' American Journal of Science/ Oct. 

 1883 ; Phil. Mag. Oct. 1883. 



