THE JURASSIC DEPOSITS WHICH UNDERLIE LONDON. 741 
VIII. Tus Great Ooxite. 
The 10 ft. of peculiar and somewhat puzzling strata which we 
have just described, rest on a series of oolitic limestones alternating 
with clayey and sandy beds, the whole having a thickness of 873 ft. 
At first sight the upper beds of this series do not appear to differ in 
a very marked manner from those lying upon them, except in the 
absence of grains of sand and glauconite. But this probably arises 
from the fact that the upper series of beds is made up of rocks re- 
constructed for the most part of materials derived from the strata 
on which they rest. 
From the examination of these oolitic strata, both in hand speci- 
mens and in thin sections under the microscope, the following 
succession of beds may be made out. It may be remarked that all 
the beds, as is so commonly the case with Jurassic limestones 
which haye not been exposed to weathering action, are of a more or 
less marked deep blue colour. This colour is due to the presence 
of iron-pyrites, which was proved by chemical analysis in the case 
of one sample of these rocks to be present in it to the extent of 2°4 
per cent. 
(1) Dark-coloured very perfectly oolitic limestone, entirely ft. in. 
made up of oolitic grains, with many small waterworn 
fragments of fossils and a few very imperfect larger por- 
tions of organisms. The upper part of this rock appeared 
to be much disintegrated and the fossils in it are quite 
BEMPOe CEPT IAD Gt sete Mas oo. til ack nee ete atte ini: 5 0 
(2) Limestones of a paler colour and more marly character, 
with scattered oolitic grains. Many Foraminifera, in- 
eluding Rotalia, Cristellaria, and Textularia, occur in this 
Tosa see eee, i cee SE Re eA MAE EL Mui Sadia t ad 22 6 
(3) Highly oolitic limestones of the same colour as (1)......... 10 0 
(4) Rock of more shelly character with fewer oolitic grains. 
Micihy PH OMAMUMILETA: 7.2.22 Joes case caenvese.<escceacc eset a eee A 0 
(5) Rock consisting of a compact calcareous paste with oolitic 
grains scattered through it. Fragments of shells, Os- 
trea, Pinna, and other Lamellibranchiata, with portions 
of Brachiopoda and small Gasteropoda and Foraminifera, 
EEPOUA WaT bs canis cu cecmeqnesniaiaces tree esse aaceusesieataas + aaeme 5 0 
(6) Beds of dark blue clay, in places crowded with fossils, and 
with subordinate limestone bands ................02000 0s eee 3 6 
(7) Limestone of oolitic and shelly character ...................4. VG 0 
(ype dr om talllers) Marth, © 22 ficlaisodatacos seins vies lois wetsaaSejsieletes 0 6 
(9) Very fine-grained oolitic limestone with much pyrites. In 
passing downwards this rock assumes a more and more 
sandy character, and at its base becomes a fissile calca~ 
reous and micaceous sandstone resembling the Stonesfield 
and Collyweston Slates (Acrosalenia and other fossils)... 9 0 
(10) Oolitic limestones with many fragmentary shells, Ostrea 
SUUCROUCR CE ans it cae totic Sanne onc noes toate eR eee eee 3 6 
(11) Band of limestone of more open texture, made up of 
large waterworn fragments of shells with a few grains 
of quartz-sand and particles of anthracite .................. 0 6 
Motall .3:4..1. Si 6 
In these limestones well-preserved fossils are very rare. The 
comnimuted shell fragments of which so many of them are largely 
Q.J.G.S. No. 160. 3D 
