Mollusca of the Laminarian Zone at Leith. 337 
by the excavations being made in the Firth of Forth at 
Leith for the New Dock Extension works, by which some 
recent shell-bearing strata are being laid bare. Here an 
area of many acres in extent has been enclosed from the sea 
by an embankment, and has been drained partly by gravita- 
tion, partly by pumping, and is now in process of excavation 
to a depth of about 25 feet under the level of low water. 
In the present notes upon the shell-bearing strata, the 
chief object kept in view is the origin of the marine 
molluscan fauna of the Firth of Forth, with especial refer- 
ence to that contained in the shelly sands just referred to. 
To this end it will be best first briefly to pass in review the 
history of such of the later physical changes the district has 
undergone, so far as these have any bearing upon the chief 
points under consideration, or as the statement of these facts 
may enable the reader more easily to grasp the significance 
of both the facts themselves, and the conclusions to be drawn 
from their consideration. 
Britain, prior to the Glacial Period, appears to have stood 
for a long time at a much higher elevation above the sea 
than it has at present. This difference in elevation may 
have amounted at one time to 600 feet, or even more. It 
was under these geographical conditions that the present 
River Forth and its tributaries, aided by the ordinary agencies 
of surface denudation, excavated the valley to something like 
its present form. Under such conditions the North Sea, at 
the time referred to, did not exist. Where that sea now is 
was then a broad plain, through which the Rhine, receiving 
as tributary streams all the rivers on the east of Britain, 
flowed northward, and discharged its waters into the Atlantic 
on the north-western side of Scotland. The margin of the 
Atlantic under these conditions coincided, or nearly so, with 
the present 100 fathom line off the west of Britain. We are 
concerned at present mainly with the fact that there was no 
sea-water where the North Sea is now; and, of course, 
equally so with the fact that there was no sea-water where 
is now the Firth of Forth. Off the west of Scotland the sea 
washed the margin of the land, and the causes which gave 
rise to the Gulf Stream then, as now, helped in the transfer 
