398 THE FOSSILIFEROUS 
lies beneath, overlaid by a thin stratum of red clay, apparently 
derived from it; but the higher-lying gray stratum in which 
the shells occur had a different origin: it is simply the partially 
consolidated mud of a quiet sea-bottom; and though its group 
of organisms manifest decidedly the boreal character, I cannot 
doubt that they lived at a time when, either from some change 
in the currents of the coast, or from the elevation of the pro- 
tecting islands outside,—an effect of a general rising of the 
land,— the sea was no longer an exposed one» They in all 
probability mark that later stage of the wintry period to which 
the last-formed group of our local glaciers belonged, and in 
which our gradually-emerging country presented, age after age, 
a broader and yet broader area, won from the deep. 
One period more, and I-shall have completed my survey. 
All the shells which have hitherto been found beneath our lat- 
est terraces of upheaval still exist on our coasts. They repre- 
sent a time, perhaps not greatly in advance of the earlier 
historic ages, when the country had begun to exist under its 
present climatal conditions. Some of these modern shells are, 
however, found to occur in very different proportions, in cer- 
tain localities, from what they do now. ‘The only specimens of 
Pholas candidus which I have been able to procure from the 
Lower reaches of the Cromarty Frith occur in a clay-bed of 
the old coast period which underlies an arable field in the 
Lones of Fern, a full mile from the sea. My only specimens 
of Serobieularia piperata from the Frith of Forth have been 
derived from the brick clays behind Portobello, more than a 
quarter of a mile beyond the reach of the tide. My first 
found Scotch specimens of Thracta convera I collected last 
year from a raised sea-bottom near North Queensferry. The 
upheaval of the land seems to have altered the conditions, in 
certain localities, favorable to the production of shells such as 
Scrobicularia aad Pholas; and Thracta convexa, though it 
