SUPPOSED CAUSES OP. 243 



respects, the more philosophical belief. Either way the 

 fact remains of its occurrence between the tertiary and cur- 

 rent epochs ; of its having brought to a close the tertiary 

 conditions that prevailed over these latitudes; and of its 

 having passed away before the present order of things 

 could be established. Wherever it occurs it forms a great 

 hiatus between the Tertiary and Recent a lifeless blank, 

 as it were, between the flora and fauna of the Tertiary epoch, 

 and the flora and fauna now flourishing around us. 



The hypotheses that have been advanced to account for the 

 glacial epoch resolve themselves into two great categories 

 first, those that depend on astronomical causes, or causes 

 extraneous to the earth ; and, second, those that depend on 

 the earth's own physical relations as regards the then pecu- 

 liar distribution of its lands and waters. This is not the 

 place to enter minutely into the merit of these theories, 

 but we may glance at their general bearings as indicating 

 the widely different sources to which geologists have been 

 driven for a solution of this strange and anomalous period. 

 By those who seek for the solution in a peculiar distribution 

 of sea and land, it is argued, that by elevating the terres- 

 trial surface in the northern hemisphere and shutting oif 

 the warm currents of the ocean from the circumpolar areas, 

 an extreme degree of cold would be induced; that the snow 

 and ice accumulated under this cold would have a tendency 

 to diminish still further the annual temperature ; and that 

 by these means the ice-sheet, in course of ages, would push 

 its way southward alike over land and sea, to the furthest 

 known verge of glaciation. This, they contend, would 

 account for the first stage of the ice-epoch : and even when, 

 during the second stage, the land began in some areas to 

 subside, the ice-sheet must have acquired so much mastery 

 over the average annual temperature as still to maintain its 



