1878.] Superficial Gravels and Clays. 341 
and the materials derived from it re-arranged, the clay being 
washed away, and the sand and gravel now constituting the 
Middle Sands and Gravels left behind. 
The explanation I have to give of this series of events is 
the same that I have offered with regard to the formation ot 
the superficial deposits of Cornwall and Devon, of the loess 
of the Danube and the Rhine, and of the diluvium of the 
South of Russia : it is — that during the Glacial period a 
ridge of ice, starting from Greenland, gradually advanced 
down the bed of the Atlantic, and ultimately blocked up the 
drainage of Europe as far as it extended. A continental lake 
was thus formed, over which floated icebergs carrying the 
northern drift. The first lake thus formed was suddenly 
discharged by the breaking away of part of the ice-barrier. 
During the tumultuous outpouring of the pent-up water the 
materials deposited during the submergence, and others de- 
rived from the pre-glacial denudation of the surface rocks, 
were caught up and spread out in great sheets over the lower 
ground, thus forming the Middle Sands and Gravels. After 
a time the ice-barrier was re-formed, the country again sub- 
merged, and the Upper Boulder-clay spread out. 
Passing on now to the consideration of the origin of the 
clays and gravels at Ealing and Brentford, the first note- 
worthy feature is the similarity in the succession of the beds 
to that of the glacial deposits at Finchley. This is brought 
out prominently in the figures by the nearly similar symbols 
under which I have grouped the divisions, but not more pro- 
minently than an inspection of the beds themselves will fully 
justify. The boulder-clay on the top of the ridge at Finch- 
ley, full of chalk fragments, is sufficiently distinct from the 
brick-clay at Ealing ; but when we descend the slopes at 
Finchley the chalky boulder-clay gradually changes to a clay 
without chalk, until, in the lower parts of the valleys, it be- 
comes a brown clay, with a pebble here and there, exactly 
similar to the brick-clays. The Middle Sands and Gravels 
at Finchley only differ from the subangular gravels of Ealing 
and Brentford in that the former contain a larger proportion 
of unbroken pebbles from the Eocene beds. This is only 
what we might expeCt on the theory of origin I have offered, 
for Finchley is nearer to the patches of Eocene gravels left 
on the hill-tops, and must have been largely made up from 
them. The greater distance the lower gravels had been 
carried, their exposure to the whole sweep of the torrents 
rushing down the Thames Valley, and their greater admix- 
ture with broken flints derived from the west, fully account 
