1878.] Glacial Drift in North America. 71 
of Europe. The second lake was gradually lowered by 
the cutting through of the Bosphorus or the Dardanelles, 
and the various stages of its subsidence are marked in all 
the great valleys of Northern and Central Europe. In 
England the upper boulder clay and the upper brick clays 
of the Thames and other river valleys were at this time 
deposited. Flint implements appear to have been found in 
clays of this age, but they do not indicate, I think, that 
palseolithic man existed, but that pre-diluvial man had left 
his nearly indestructible stone-work on the hill-sides, higher 
than the violence of the debacle reached to, and that shore- 
ice in the second-lake period sometimes carried these away 
and dropped them in the clay that was then forming. I no- 
ticed, in Dr. John Evans’ noble collection of stone imple- 
ments, with surprise, a faCt with which he had been long 
familiar — the sharp, unworn edges of the implements from 
the brick clays, and also of a few that have been found on 
the surface at heights of over 300 feet above the sea. These 
implements have also a whitened bleached appearance, which 
may be due to long exposure on the surface before being 
imbedded in the brick clays. 
It is now more than two years since I laid this theory 
before the Geological Society of London,* and no flaw has 
yet been pointed out in it, whilst in a series of papers pub- 
lished in this Journal I have shown that many other difficult 
problems in glacial geology besides those to the solution of 
which I first applied it find in it a simple explanation. I 
believe it is the only theory that explains the transport of 
northern boulders across the plains of Germany and Russia ; 
and at the same time accounts for the absence of marine 
remains testifying that the sea had not occupied the area 
during the flotation of the blocks ; and the absence of glaci- 
ated rock-surfaces showing that the Scandinavian land-ice 
had not extended so far. It is also the only theory of our 
day that deals with the difficult question of the origin of a 
great debacle of which De la Beche, Murchison, Sedgewick, 
and Prestwieh have shown us there is so much evidence. 
The principal feature in the theory is that the advance of 
the ice of the Glacial period was mostly down the ocean 
depressions, partly because ice will gravitate towards the 
lowest levels, and especially because the precipitation of 
moisture is in our hemisphere much greater at the northern 
ends of the seas than in similar latitudes inland on the 
continents. 
* “ Drift of Devon and Cornwall.” Read November 3rd, 1875. Published 
in abstract only, Quart. Journ. Geol. Soc., February, 1876. 
