1891.3 
263 
[Shaler. 
It is evident that something of the same objection which has 
been made to the other physical indices of post-glacial time ap- 
plies also to this evidence obtained from the shore. We are in 
entire ignorance as to the amount of the elevation above the 
present coast line which has existed. We only infer that the land 
has been higher than at the present day from the evidence of 
submerged forests and swamps. There may have been half a 
dozen benches formed below the present plane of the sea. Never- 
theless, the positive evidence of the lapse of time since the disap- 
pearance of the ice from the shore leads us to the opinion that 
any such duration as 10,000 years is quite insufficient to account 
for the observed facts. 
Probably the most trustworthy evidence concerning the dura- 
tion of the time since the ice sheet began to disappear from the 
land is afforded by the distribution of the vegetation, which by 
migration has regained possession of the glaciated district. It is 
evident that a considerable time was required for the return of 
any forms of phenogamic plants to the field laid waste by the con- 
tinental glacier. Even in Switzerland where the retreating ice 
has recently exposed extensive accumulations of morainal matter, 
very near to abundant vegetation the plants only slowly secure a 
foothold upon the drift. The greater portion of our flowering 
species require a certain preparation of soil which is brought 
about by the action of cryptogamic forms before their roots can 
find adequate nourishment. We must conceive that as the ice 
retreated and gradually disappeared from the surface, a consid- 
erable time must have been required before the existing forests 
could have attained their organization. 
Besides the general impression as to the need of great time for 
the restoration of the plant envelope to the glaciated field we may 
obtain from a study of particular species certain approximate 
time ratios, which seem to me to afford proof that at least one 
hundred thousand years, and possibly a much greater time, has 
elapsed, since the ice began to disappear from the central portion 
of the continent. This evidence we obtain from the rate at which 
our large seeded species of trees can march into a new country. 
Where the seeds of a plant are small, as is the case with the wil- 
lows and birches, strong winds may carry them to great distances ; 
or they may be entangled in the mud which becomes affixed to 
the feet of water-loving birds and be conveyed in the migrational 
