The Geographical Journal (Journal of the Royal Geographical Society): Volume XII, No. 2 [‎269r] (40/154)

OCEAN, AND ITS RELATION TO OTHER OCEANOGRAPHICAL PHENOMENA. 131
■and elsewhere, while in the specimens from the deep sea this base is
never present ; only the hard shell of dentine remains. *
The long lines of phosphatic nodules which occur, for instance, in
the calcareous Miocene and Oligocene beds of the island of Malta f thus
possibly indicate the occasional destruction of a large amount of animal
life in the surface waters of the Tertiary ocean, owing to changes of
temperature, although it might also be due to changes of salinity and
other causes.
On the Agulhas bank, the Challenger and the Gazelle met with very
characteristic glauconite deposits in depths between 100 and 150 fathoms,
and the phosphatic nodules in these deposits were of a dark-green
colour. An analysis of the Challenger material gave 19*96 per cent, of
phosphoric acid, and 39 , 41 per cent, of lime; and an analysis of the
Gazelle material gave 38 88 per cent, of phosphate of lime. In the same
region the Challenger dredged similar nodules of a lighter colour in
1900 fathoms. It is probable that the phosphate in these nodules was
ultimately derived from the fish and other animals killed at the surface,
and in shallow depths, by sudden changes in the temperature conditions,
this being one of the areas of wide range indicated on the accompanying
map.
Still another effect produced in the deposits by sudden changes of
temperature at the surface is that the deposits of Globigerina Ooze appear
to be much purer beneath those areas where currents from different
sources meet at the surface, especially far from coasts. This, in all
probability, arises from the more abundant destruction of these pelagic
1 oraminifera in such situations than in other areas with a more con
stant temperature. The rain of the carbonate of lime shells would be
more rapid and constant, and consequently they would bulk more largely
in the formation of the deposit compared with the inorganic materials
which are usually present. In this way there may possibly be a more
rapid formation of deposits beneath areas with a large annual range
than at other places.
It has already been stated that the circum-equatorial zone, with a
high temperature and small range, includes the typical coral-reef
regions of the world. In the tropical surface waters there is a great
development of organisms which secrete carbonate of lime, such as
pelagic molluscs and Foraminifera, coccospheres and rhabdospheres, and
in the shallow depths around the shores organisms with massive car
bonate of lime shells and skeletons are abundant, such as corals, decapod
crustaceans, and a great variety of molluscs and calcareous algse. As
we proceed to the colder waters of the north and south the organisms
* Murray, “Eeport on the Blake Deposits” {Bull. Mus. Comp. Zool, vol. xii. p. 42).
1885.
f Murray, I he Maltese Islands, with Special Reference to their Geological Struc
ture” {Scott. Geogr. Mag., vol. vi. p. 449). 1890.