178 
ELEMENTS OF GEOLOGY. 
from Caithness, and the same number from Aberdeenshire 
and Banff, and in both cases all but six are arctic species. 
I formerly suggested that the absence of all signs of or¬ 
ganic life in the Scotch drift might be connected with the 
severity of the cold, and also in some places with the depth 
of the sea during the period of extreme submergence; but 
my faith in such an hypothesis has been shaken by modern 
investigations, an exuberance of life having been observed 
both in arctic and antarctic seas of great depth, and where 
floating ice abounds. The difficulty, moreover, of accounting 
for the entire dearth of marine shells in till is removed when 
once we have adopted the theory of this boulder clay being 
the product of land-ice. For glaciers coming down from a 
continental ice-sheet like that which covers Greenland may 
fill friths many hundred feet below the sea-level, and even 
invade parts of a bay a thousand feet deep, before they find 
water enough to float off their terminal portions in the form 
of icebergs. In such a case till without marine shells may 
first accumulate, and then, if the climate becomes warmer 
and the ice melts, a marine deposit may be superimposed on 
the till without any change of level being required. 
Another curious phenomenon bearing on this subject was 
styled by the late Hugh Miller the “ striated pavements ” 
of the boulder clay. Where portions of the till have been 
removed by the sea on the shores of the Forth, or in the in¬ 
terior by railway cuttings, the boulders embedded in what 
remains of the drift are seen to have been all subjected to a 
process of abrasion and striation, the stria© and furrows be¬ 
ing parallel and persistent across them all, exactly, as if a 
glacier or iceberg had passed over them and scored them 
in a manner similar to that so often undergone by the solid 
rocks below the glacial drift. It is possible, as Mr. Geikie 
conjectures, that this second striation of the boulders may 
be referable to floating ice.* 
Contorted Strata in Drift, —In Scotland the till is often 
covered with stratified gravel, sand, and clay, the beds of 
which are sometimes horizontal and sometimes contorted for 
a thickness of several feet. Such contortions are not uncom¬ 
mon in Forfarshire, where I observed them, among other 
places, in a vertical cutting made in 1840 near the left bank 
of the South Esk, east of the bridge of Cortachie. The con¬ 
volutions of the beds of fine and coarse sand, gravel, and 
loam, extend through a thickness of no less than 25 feet ver¬ 
tical, or from 5 to c, Fig. 115, the horizontal stratification 
being resumed very abruptly at a short distance, as to the 
* Geikie, Trans. Geol. Soc, Glasgow, vol. i., part ii., p. 68. 1863. 
