Mat 1902.] BOTANICAL SOCIETY OF EDINBURGH 247 



as well, even where the ice was thin. The consequences 

 of such a depression were twofold : tirstly, the sub- 

 sidence gradually admitted the sea where there had 

 formerly been land, and thus tended to ameliorate the 

 climate ; and, secondly, the depression lowered the moun- 

 tain tops to an elevation much nearer to the sea-level,, 

 where it is warmer than it is at higher elevations. 

 As a consequence, the precipitation began to take the 

 form of rain instead of snow. So the glaciers were cut 

 off at their source, and they quietly and slowly melted 

 away as they stood, leaving the stones and mud with which 

 their lower strata were charged, as a kind of sedimentary 

 deposit, which gradually accumulated between the ice and 

 the underlying rock in the case of the boulder clay ; and 

 which, where crevasses existed, gave rise, by the washing of 

 sand and gravel from the surface of the ice into these fissures, 

 to those remarkable deposits of sand and gravel which are 

 generally known as Eskers, or as Kaims, in Scotland.^ 



The sea was admitted up all the old river valleys, such, 

 for example, as the Forth, the Clyde, the Tay, and others 

 in Scotland, as well as in other cases in various other parts 

 of the kingdom. Drowned river valleys, with the tops of 

 the smaller hills standing up as islands, occur not only in 

 the Forth, but all round the British Islands. 



It is important for our present purpose to remember 

 that the places which would be the earliest to be dis- 

 encumbered of their ice and snow, under the depressed 

 conditions, would be the parts most remote from the great 

 mountain masses, and especially those in the southern 

 parts of the kingdom. Xext in order to these were the 

 hilltops themselves. Therefore the species of plants earliest 

 to resain a footing; would do so in either what were the 

 maritime areas for the time being, or else in the newly 

 exposed alpine region of the uplands. 



It seems highly probable (though it by no means follows 



that it was really the case) that, as the clirnatal conditions 



became suitable, and as congenial soils and habitats became 



1 It is but fair to myself to mention here tiiat tliis explanation of 

 the eiiglacial original of boulder clay, and the formation of Eskert;, 

 was first put forward by myself in the "Geological Magazine," for 

 November 1874. Other writers have repeatedly put it forward as new 

 since then. 



