2 
POPULAR SCIENCE REVIEW. 
We must, however, determine their relations one to another, and 
this can he done in one way by obtaining the records of some 
of the London deep-wells ; and in another way by studying the 
numerous pits and railway-cuttings, which frequently show the 
junction between two or more of the different rocks. Classifying 
the observations so made, we find the following is the order in 
which the strata occur ; and although of course all are not 
continuous over the area, this order is never inverted : — 
Alluvium , ill Thames Valley deposits. 
Gravel, sand, and brickearth ) J r 
Boulder clay j Qq a( q a i deposits. 
Gravel and sand ) r 
Lower Bagshot beds. 
London clay. 
Oldhaven beds. 
Woolwich and Beading beds. 
Thanet beds. 
Chalk. 
Thus we find from well-sections that the chalk is everywhere 
present at great depths beneath London. (See Section.) Again, 
in whatsoever direction we go by rail on the main-lines, we 
come to the outcrop of the chalk, as near Ipswich, Bishop’s 
Stortford, Hatfield, St. Albans, Watford, Maidenhead, Basing- 
stoke, Guildford, and Croydon. The chalk, therefore, crops up 
on the north, west, and south of London, and forms a sort of 
basin in which the London clay and other beds now lie. Hence 
this geological area is called the “ London Basin.” 
The chalk is usually divided into the upper chalk with 
flints, the lower chalk without flints, and the chalk marl ; the 
total thickness of which is estimated to be about 500 feet at 
Croydon. 
The pits at Charlton, Gravesend, and Grays are no doubt 
well known to Londoners ; whilst the hollow flints containing 
sponges, the “ sugar-loaves,” or sea-urchins, and the “thunder- 
bolts ” (as sometimes the Belemnites, at others the nodular 
masses of iron-pyrites, are called), which are found in the chalk, 
are equally familiar. Besides sponges and sea-urchins, the 
chalk yields many forms of ammonites, oysters, and other 
mollusca, a few corals, a number of fish and reptiles, amongst 
which the Mosasaurus is estimated to have been 24 feet in 
length. But the most abundant fossils found in the chalk, and 
of which indeed it is to a great extent made up, are the Fora- 
minifera, minute many-chamber ed shells, which belong to the 
lowest class of the animal kingdom. “ The best method of 
finding them is to scrape a small quantity of the chalk to be 
examined very finely with the penknife, and after moistening it 
