2 BOROUGH OF LEWISHAM. 
CHAPTER I. 
THE ‘‘So.ip” GEOLOGY. 
HE lowest formation exposed in our district is the 
chalk which is the uppermost member of the 
; Cretaceous system, that system again concluding 
NEF) the*Secondary or Mesozoic Geological Period. A 
greyish mud is now in course of deposition on 
the floor of the North Atlantic which is known as 
the Globegerina ooze, the Globegerine being a large genus of the 
order Foraminifera. Probably this ooze will in some long distant 
future constitute the chalk formation of a yet unborn continent. 
Although most of the neighbouring chalk is protected by the 
overlying Lower Tertiary beds, which will be presently described, 
it is doubtful whether any of it represents the original uppermost 
zone. In Mr. Whitaker’s ‘‘Guide” mentioned above it is stated 
that the greatest thickness of chalk disclosed by well-borings near 
London is 685ft. at Bushey. At Margate a thickness of 7o04ft. has 
been bored through. We believe that at Dover it reaches 1,oooft. 
It has been estimated that the Globegerina ooze is now being 
deposited at the rate of a foot in a hundred years. If the chalk 
was deposited at the same rate, then 100,000 years was occupied 
in the deposition. 
Many theories have been suggested in regard to the origin 
of the noduiar flints which occur in bands in the upper chalk. 
Let it be premised that water charged with carbon-dioxide derived 
from the atmosphere is a powerful soivent. All rivers are con- 
tinually carrying to the sea mineral matter of various descriptions 
held in solution. Carbonate of lime is one such mineral, and with 
it the Foraminifera form their shells. Silica is another mineral 
which is used by the Radiolaria, another order of microscopic 
Protozoa. It is also used by Sponges for their sfzcula. As- 
suming that the waters of the cretaceous ocean contained at 
alternating periods a preponderance, first of carbonate of lime, 
and then of silica, the Foraminifera or Radiolaria would alternately 
abound. They died in millions, and their tests sunk to the sea 
floor. But the tests were again dissolved, and the silica gathered 
about dead organic matter and nodules of chalk. Atoms of silica 
penetrated the dead organisms and the chalk nodules, and dis- 
placed the carbonate of lime. Thus fossil echinoderms are some- 
times found, consisting mainly of carbonate of lime, and others 
entirely of flint. Tabular flint, of which masses may be seen in 
some of the galleries of the Chislehurst Caves, was probably 
deposited after the chalk had become indurated. Water, charged 
with silica, penetrated between the layers of chalk, and there the 
silica came to rest. From the impress of organic structures some- 
times seen in flint, it is inferred that at an early stage the silica 
formed jelly-like masses, which ultimately became harder than steel. 
