OF THE PARALLEL ROADS OF LOCHABER. 
693 
The effects due to these two causes would certamly amount to a difference in the 
two periods of not less than from 12° to 15° Fahr., which is equivalent to the difference 
now existing in annual temperature between London and the south coast of Spain 
or of Sicily, and to more than exists between St. Petersburg and Paris.'" 
There is also to be taken into consideration the increase of heat consequent on the 
gradual diminution of those intense glacial conch dons' 1 '—holding the cause not to be 
entirely physiographical—which has ended in our present climate. 
Supposing the subsidence to have been slow (though there is no reason to suppose 
that it was at all so slow as that of Scandinavia or Greenland at the present day), and 
that the sea crept gradually and quietly over the land, slowly floating away the ice 
covering the submerged parts, there would have ensued a gradual and steady rise of 
temperature raising the level of the snow-line, and resulting in a steady and continuous 
thaw. The great ice-sheet of Scotland, with its frequent small gradients, conflicting 
elements, and its many projecting mountain ridges, must at this time have presented a 
surface of great irregularity. The water would, as the thaw proceeded, collect in pools 
and ponds, and, in the absence of existing water channels, would there remain until 
the further progress of the thaw levelled the obstacles presented by the uneven surface 
of the ice, and allowed it to escape either by the wear of channels on the surface or by 
crevices in the body of the ice, which however from the small gradients and the 
pressure in the main valleys must have been generally too compact for the latter 
purpose. Such ponds and lakes are, in some places, where glaciers descends into 
warm valleys below the snow-line, of not uncommon occurrence, and under certain 
circumstances lakes of considerable size are thus formed. 
In glacier districts there are four classes of lakes—the first are those formed by the 
greater glacier of a main valley crossing, at some distance below the snow-line, the 
entrance of lateral valleys, the streams in which they thus dam back; the second by 
the smaller glacier of a lateral valley traversing the main valley and forming a barrier 
to the head waters of a river; the third by water accumulating in hollows on the 
surface of the glaciers themselves, and there forming pools and small lakes; and the 
fourth those where streams have been dammed back by old terminal moraines. With 
this last description of glacial lakes we are not at present concerned. 
In the first class of lakes, of which the Merjelen See is an instance, the water escapes 
from time to time with greater or lesser rapidity through fissures in the glacier, leaving 
the barrier otherwise intact and soon again available ; the second class gives rise to 
Scandinavia. The difference in the winter temperature of Bergen and Stockholm, which are nearly in 
the same parallel of latitude, is, according to Dove, 10° 28’ ; while the relative difference in the winter 
and summer temperatures of these places is 21° 28' and 34° 39' Fahr. 
* Comparing the winter temperature of Scotland (taken at 38°) with that of Greenland (taking the 
mean between Lichtenau in 60° 31' N. lat., and Upeimivik in 72° 48' N. lat.), they are as 38° to 5° Fahr., 
or a difference of 33°; but the mean annual temperature of Greenland, taken in the same way, gives 23°, 
and that of Scotland about 47°, or a difference of only 24°. 
+ None of the reasons hitherto assigned for these conditions are to my mind satisfactory. 
