6 
POPULAR SCIENCE REVIEW. 
the palm, and ferns then grew), the London area was probably 
dry land. The action of rain and rivers in wearing away the 
land then came into operation, and probably the main features 
of this part of the country were formed. Subsequently, during 
the pliocene period, parts of the eastern coasts of Norfolk, 
Suffolk, and Essex came within the influence of the sea, which 
has left its traces in the deposits of shelly sand called crag. 
The evidence furnished by the mollusca in these deposits shows 
a gradual lowering in the temperature, while remains of 
mastodon, elephant, tapir, rhinoceros, hippopotamus, bear, and 
musk-ox are found in some of the later deposits of the period. 
We now come to treat of deposits of which we find traces to 
the north of London, and which indicate that a glacial climate 
prevailed over the northern hemisphere. In the railway- 
cuttings at Bricket Wood (between Watford and St. Albans), 
and at Finchley, also in pits at Muswell Hill, we find a clayey 
deposit containing flints, numerous pellets and boulders of 
chalk, and other rocks, and fossils from almost every geological 
formation in England. This is called the boulder clay, and 
it overlies in places beds of sand and gravel, formed of rocks 
which have come from far distant parts, and containing 
fossils likewise derived from older and distant geological 
formations. Indeed, from these deposits at Muswell Hill and 
the neighbourhood, Mr. Wetherell, of Highgate, made a very 
extensive collection of British fossils. These sands and gravels, 
called middle glacial by Mr. Searles Wood, jun., have been 
proved to contain some organic remains which lived at the 
time of their deposit. They comprise over sixty forms of 
marine shells, which he has collected near Yarmouth. The 
boulder clay owes its origin to very different conditions. The 
presence of the rolled chalk in such abundance can be due to 
no other agency than moving ice, according to Mr. Wood. 
The degrading influence of a vast ice-sheet which covered the 
northern counties would have formed an immense quantity of 
material, and this being gradually expelled at the margin of 
the sea which then stretched over the south of England, it was 
washed away and dispersed over the midland and eastern 
counties, where we now find it. 
The country must then have been in a condition resembling 
the polar regions, where vast sheets of ice, due to the accumu- 
lation of snow on the heights, press downwards into the 
lowlands, and so onward to the sea, to break off and form 
icebergs, bringing with them and dispersing much clayey, 
sandy, and gravelly material and boulders. Mr. Wood points 
out, that from the middle glacial deposits to the boulder clay, 
although there was such an abrupt change in character, there 
was an uninterrupted succession of deposit. 
