APPENDIX. 44 1 



Note VII., p. 228. 



I have given in the text the usual explanation of the phenomena of 

 the glacial epoch. The theory of northern elevation, however, as the sole 

 or principal cause of continental glaciers, has never been regarded, by 

 many geologists, as completely satisfactory. Within a few years renewed 

 attempts have been made to connect these phenomena with astronomical 

 changes of a secular character. 



At sundry epochs in the history of the world, agencies seem to have 

 arisen which brought into existence and transported over considerable dis- 

 tances vast quantities of rounded pebbles and finer detrital materials. In 

 some cases — as in the Niagara, Permian, and Upper Miocene periods — 

 smoothed and striated rock-surfaces have been discovered, similar to those 

 which are generally attributed to glacier action. In the intervening pe- 

 riods evidences of tropical temperature present themselves. The sugges- 

 tion has therefore been made that more than once in the history of the 

 world — perhaps at somewhat regular intervals widely removed — the north- 

 ern portions of the continents have been visited by a reign of frost. 



To account for these apparently secular phenomena, new investigations 

 have been made upon the effects of the secular variations in the longitude 

 of. the equinox, the eccentricity of the earth's orbit, and the obliquity of 

 the ecliptic. This is not the place to enter into an exposition of the 

 discussions which have arisen. I may, however, simply explain the na- 

 ture of the relation which subsists between terrestrial climates and the 

 cosmical changes alluded to. 



1 . As to the variation in the eccentricity of the earth's orbit, it is evi- 

 dent that when the northern hemisphere has its winter in perihelion dur- 

 ing the time of greatest eccentricity, the amount of glaciation must be 

 considerably less than when the same hemisphere has its winter in aphe- 

 lion during the time of greatest eccentricity. 



2. As to the variation in the obliquity of the earth's axis, it appears 

 that when the obliquity is greatest, the winter temperature of the polar 

 regions can not be much severer than when the obliquity is least — since 

 when the sun is below the horizon it is immaterial whether it be two de- 

 grees or ten below — while the summer temperature of the polar regions 

 will be increased by the whole increase in the verticality of the sun's rays. 

 The effect, therefore, of an increase in the obliquity of the earth's axis 

 will be to diminish the average glaciation of the polar regions. 



3. Suppose now the minimum glaciation of the polar regions, so far as 

 due to obliquity, to occur at the time when the northern hemisphere ex- 

 periences minimum rigors of climate through the effect of increased ec- 

 centricity ; the conjunction of these two minima of cold in the north polar 

 regions would, it is thought, remove the ice cap, and effect conditions of 

 climate such as prevailed when Greenland, in the Miocene period, support- 

 ed trees of tropical nature and luxuriance. 



T2 



