20 A Period in the History of our Planet. 



no proof that an icy burden of 8000 feet in thickness rested 

 upon the plain of Switzerland, the provinces of Germany, &c. ; 

 retrenching and deducting as much as we can find possible, 

 in order not to terrify our imagination with the grandeur of 

 the object, there will still remain something immense, to which 

 our knowledge of the present world would in vain seek to pre- 

 sent any thing parallel or similar. 



It would, however, in my opinion, be an erroneous concep- 

 tion of the consistence of this ice-crust, to suppose it to be com- 

 posed of a solid mass. Where there prevailed such an uncom- 

 monly low temperature as was necessary to envelope the earth 

 to such an extent in a frozen covering, all the necessary ele- 

 ments were wanting to change the loose deposits of the at- 

 mosphere into compact ice. For it is only by the addition of 

 fluid water, by the repeated alternations of the thawing and 

 freezing of the snow-masses saturated with water, that these 

 become gradually transmuted into solid ice, which, however, 

 still bears the traces of its origin. 



In our high Alps, it is only to a certain height that we 

 meet with compact ice ; above that height we find only loose 

 snow, or what the natives call Firn, the loose incoherent mass 

 of which leaves no trace of its existence upon the rocks. This 

 is simply owing to the circumstance, that above these boun- 

 daries — the Firn-Une — the temperature never attains for any 

 length of time to such a height as is requisite for the produc- 

 tion of fluid water, and so for the formation of solid ice. But 

 in an epoch of general refrigeration like that of the glacial 

 period, the earth collectively came into a state of temperature 

 which must have been analogous to that of the Alps at the 

 present day, that is, the Firn-line descended in proportion to 

 the refrigeration, and probably so low, that, in the tempe- 

 rate zones at least, even at the level of the sea there existed 

 only snow or Firn, but no ice, the temperature, standing almost 

 constantly below 32° Fahr., not being sufiicient to call forth a 

 formation of solid ice by the melting of the upper layers of 

 snow. 



But if such a partial and superficial thawing of the mea- 

 sureless snow-fields of the glacial period, and their transition 



