Mollusca of the Laminarian Zone at Leith. 339 
is increased in proportion to the thickness and extent of the 
mass. In Scotland, the whole country was enswathed in ice 
at the climax of the Glacial Period, which ice could not 
have had a thickness less than 2000 feet around Edinburgh, 
and must have been even considerably thicker than that 
nearer its source. 
Many physicists are of opinion that the earth’s crust 
everywhere is always in a condition of unstable equilibrium, 
ready to rise if a load is eased off it at any part, equally 
ready to sink if a load is placed upon it. Hence the loading 
of this part of the earth’s surface by a heavy mantle of ice 
may well have resulted in a slow and gradual subsidence. 
Whether the cause alluded to is or is not the correct one, 
is not very material for the present purpose. But it is 
certain that. after the climax of the glacial conditions was 
reached, the land here began to subside. As I hold that the 
Gulf Stream was in operation all through the Glacial Period 
(and, indeed, was the primary source of the snow by which 
the glaciers and the ice-sheet were nourished), it follows that 
with each foot of subsidence an increasingly-larger area was 
covered by the sea, and a rapid amelioration of climate 
ensued in consequence. In the meantime, cold surface- 
waters still favoured the southward migration of the /ry 
of Arctic Mollusca (and other marine invertebrates). There- 
fore, as the land subsided, and the sea gained admittance to 
the area now occupied by the German Ocean, communita- 
tion was established with the Atlantic and the old valley 
of the Forth, into which the sea for the first time gained 
admittance, while the southern entrance to the German Ocean 
was barred by the chalk hills (since broken through) between 
Dover and Calais. 
The first marine fauna of the North Sea, and therefore 
of the Firth and the estuary of the Forth, gained admittance 
as fry borne along by surface currents entering this area 
from the region between Scandinavia and Shetland. Such a 
fauna had necessarily an Arctic facies, and could not well 
include many Celtic or Lusitanian forms of Mollusca, such 
as held their own, perhaps through the whole of the Glacial 
Period, in the area to the west of Britain. 
VOL, XIII. 2A 
