MIDDLE CHALK—CONDITIONS OF DEPOSIT. 
553- 
In oceanic deposits the dimensions of such particles are also 
variable, and the range of size is not very different from the above. 
The following particulars are taken from those given in the Report 
on Deep Sea Deposits : — 
Mineral Particles in 
Range of size. 
Average size. 
mm. 
mm. 
Globigerina oozes - 
•06 to *8 
•089 
Pteropod oozes ----- 
•06 to -1 
*08 
Green muds ----- 
*06 to •2 
*13 
Coral muds - - - - 
very small 
•07 
Blue muds. 
•06 to *3 
•115 
It would seem, therefore, that no reference as to depth can 
be drawn from a consideration of the mineral particles found 
in the Middle Chalk. The only inferences deducible are that 
it was formed in clear water of some depth at a considerable dis¬ 
tance from land and in a region where there were no volcanoes. 
Let us now see if the organic contents of this division can give 
us more definite indications. In the first place, we must point 
out that the fauna is rather a small one, and that for some reason 
or other the conditions were not at this time very favourable 
to animal life, but this very scarcity of fossils may in itself be signi¬ 
ficant. We will refer to the different classes of animals separately. 
Fish. —Little can be learned from the remains of fish ; the 
commonest are those of sharks, which range all over the oceans, 
and are found in all kinds of deposits. Even Teleosteans, like 
the flat-fish which live on the bottom, have a great, range in depth, 
some species being found below 1,000 fathoms, but Dr. Smith- 
Woodward remarks that “ the Teleostean fish of the Chalk do 
not appear to have lived in very deep water, for the skeleton is 
always more robust than that of the true deep-water fish.”* A 
possible explanation of the presence of such fish in the Middle 
Chalk has, however, been suggested by Dr. Hume, that they 
followed food carried out by a current setting from the shore.f 
Mollusca, —The Molluscan fauna is noteworthy because it 
consists almost entirely of Brachiopoda , Lamellihranchiata and 
* Quoted by Cayeux in Contribution a 1’etude Micrographique, p. 500. 
f Proc. Geol. Assoe., vol. xiii., p. 233. 
j. j 
