GEOLOGICAL PRINCIPLES. 129 



are found in New Zealand, Tasmania, and the south- 

 ern portion of the Andes; but whether this cold period 

 was coincident with that of the Northern Hemisphere 

 we have at present no means of determining, nor even 

 whether they were coincident among themselves, since 

 it is quite conceivable that they may have been due to 

 local causes, such as greater elevation of the land, and 

 not to any general cause acting throughout the south 

 temperate zone. 



In the north temperate zone, however, the phenomena 

 are so widespread and so similar in character, with only 

 such modifications as are readily explained by proximity 

 to, or remoteness from, the ocean, that we are almost sure 

 they must have been simultaneous, and have been due to 

 the same general causes, though perhaps modified by 

 local changes in altitude and consequent modification of 

 winds or ocean-currents. The time that has elapsed 

 since the glaciation of the Northern Hemisphere passed 

 away is, geologically, very small indeed, and has been 

 variously estimated at from 20,000 to 100,000 years. 

 At present the smaller period is most favored by geolo- 

 gists, but the duration of the ice age itself, including 

 probably one or more inter-glacial mild periods, is ad- 

 mitted to be much longer, and probably to approach the 

 higher figure above given. 



The undoubted fact, however, that a large part of the 

 north temperate zone has been recently subjected to so 

 marvellous a change of climate, is of immense interest 

 from many points of view. It teaches us in an impres- 

 sive way how delicate is the balance of forces which ren- 

 ders what are now the most densely peopled areas habit- 

 able by man. We can hardly suppose that even the 



