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, which, 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 fi^om 
the gravels of the fourth and sixth periods, and, indeed, there is 
reason to beheve, 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. 
