84 Capt. F. W. Hutton on the Origin of the 



partly filled by marine Upper Cretaceous rocks, upon which 

 lie Oligocene and Miocene strata. 



What now has been the cause of these three glacier epochs 

 in New Zealand ? Are we to attribute each of them to a 

 general lowering of the temperature of the southern hemi- 

 sphere — that is, to a true glacial epoch ? Or are we to refer 

 them to some other cause? According to Mr. Wallace our 

 last glacier epoch was due to a general lowering of the tem- 

 perature, brought about by changes in the eccentricity of the 

 earth's orbit, in combination with geographical changes, as 

 explained by Dr. Croll. It will therefore be necessary to 

 say a word on this subject. 



It is well known that, owing to the varying attractions of 

 the planets, tlie mean annual velocity of the earth in its orbit 

 is not the same year by year ; and as the earth has to com- 

 plete its annual revolution round the sun in a fixed time, the 

 distance it travels in each year varies. When the mean 

 velocity increases the orbit increases, and vice versa. But as 

 the length of the major axis of the orbit must remain constant, 

 a greater or less length of orbit is obtained by an increase or 

 decrease of the minor axis. So that when the average speed 

 of the earth is great the orbit becomes more nearly circular, 

 and when the average speed is small the orbit becomes more 

 oval. But the sun must always occupy one of the foci of the 

 orbit. Therefore as the orbit gets flatter the position of the 

 sun continually recedes from the centre ; or, in other words, 

 the orbit gets more and more eccentric. As the annual 

 amount of heat received by the earth from the sun varies 

 inversely as the length of the minor axis, it follows that the 

 greater the eccentricity the greater is the total amount of heat 

 received from the sun. But when the eccentricity is great 

 the earth is much nearer the sun in perihelion and further 

 away from it in aphelion than when the eccentricity is small. 

 Consequently that hemisphere of the earth which has its 

 winter in aphelion and its summer in perihelion during a time 

 of great eccentricity will have its seasons exaggerated, a 

 long and very cold winter being followed by a short but very 

 hot summer ; while the other hemisphere will have a short 

 warm winter followed by a long and cool summer. Owing, 

 however, to the combined action of precession of the equi- 

 noxes and revolution of the apsides, the hemisphere which 

 has its winter in aphelion is changed every 10,500 years, and 

 as a period of great eccentricity will last longer than this, 

 these alternations of climate will recur perhaps three or four 

 times before the eccentricity is greatly diminished. 



