226 



THE GEOLOGIST. 



aided by examination of that icy fringe of the northern zone, which 

 has of late so materially influenced our climate, as if we were again 

 menaced with a southerly extension of polar ice. 



The geological value of the study of Icelandic glaciers is well set 

 forth by Mr. Longman, when pointing out the heaps of sand and 

 clay bedded in their substances, Avhich, when the progressive motion 

 of the glacier from the jokull or ice-mountain is stayed, are seen to 

 form " catenation of small hills round its base" — features in the 

 natural arrangement of surface-material to be paralleled in the 

 mountain -districts of Wales and Cumberland. 



And although these histories of arctic and sub-arctic conditions 

 come in at the close of the geological record, yet they are by no 

 means insignificant in their operations, nor were they slight in their 

 duration. 



The Pleistocene age of Scotland is shown by the researches of Mr. 

 Chambers to contain within its limits seven periods, marked by 

 distinct deposits, each the result of an important physical alteration 

 of surface-aspect. The descending order of these, ending with the 

 deposition of the boulder-clay which inaugurated arctic conditions, 

 is thus stated by him : — 



1. Vegetable soil — mosses. 



2. Ancient sea-margins — erratic blocks from sub-aerial glaciers. 



3. Ancient valley-glaciers and moraines. 



4. Beds of sand and gravel. 



5. Upper boulder-clay, marking a short but violent sub-aqueous 



glacial drift. 



6. Deposit forming brick-clay, with sandy beds and gravel. 



7. Boulder-clay; laid by sub-aqueous glacial conditions, with 



moraines of ice. 



Most of these were continued into England, or have their equiva- 

 lents there, and are now, from the nature of their contents, attract- 

 ing the chief attention among geological observers. The ancient 

 flint-weapons and implements fashioned by human hands come from 

 the gravels of the fourth and sixth periods, and, indeed, there is 

 reason to believe, were in use by human inhabitants of high grounds 

 during the seventh or true boulder-clay period, at the time that 

 wide-spread deposit was being laid in the valleys. 



