26 A Period in the History of our Planet, 



towards the Alps began, these blocks were moved thence to- 

 wards their present situations. 



I shall here be accused of a contradiction. «' How is it pos- 

 sible,'* it will be said, " that the ice-covering retired just to- 

 wards the points from which the blocks were moved ? How 

 can it have happened that these were conveyed from the centre 

 of the chains towards the periphery of the ice-fields, whilst 

 their bearers, the masses of ice, had a retrograde motion from 

 the periphery to the centres — that is, in a direction diametri- 

 cally opposite to that of the blocks f' 



There lies, no doubt, an apparent contradiction in the truths 

 just announced, but it is only apparent ; for the retreat of a 

 mass of ice is nothing but the result of a disproportion be- 

 twixt the forces which tend to its destruction on the one hand, 

 and its forming elements on the other, in which the former at- 

 tain the upper hand. Snow and water are the forming ele- 

 ments, upon the co-operation of which the production and pro- 

 gressive motion of the glacier masses depend ; by the constant 

 addition of the former the totality of the latter is increased, 

 and this increase, united to the expansion which the freezing 

 water undergoes, are the causes of the progressive motion, 

 which is properly only an expansion of the mass in a single 

 direction. But if the melting and evaporation of the masses 

 of ice which are occasioned by the heat outweigh the expan- 

 sion, if more is withdrawn from the mass in a fluid and 

 vapoury form than it gains in the way of condensation of the 

 atmospheric deposits, it must obviously decrease in bulk, and 

 its decrease will be most where these destructive influences 

 operate most strongly ; that is to say, at a distance from the 

 colder centres, at the extreme periphery of the masses, where 

 these come in contact with a warmer air and soil. It is ac- 

 cordingly this preponderance of the forming influences over the 

 destructive in the glacier masses, which, to the eye of the su- 

 perficial observer, appears as the progressive motion ; and in 

 the same way it is the depression of the forming elements, 

 which appears as the retreat of the masses ; whilst, on a more 

 exact observation of the phenomena, it is found that the mass 

 constantly moves forward, even when a greater decrease at 

 particular places appears there necessarily to imply a retreat. 



