254 
Proceedings of the Royal Society 
Some of the substances in solution, as carbonate of lime and silica, 
are extracted by animals and plants to form their shells and ske- 
letons ; these last, falling to the bottom, form a globigerina, a pte- 
ropod, a radiolarian, or a diatom ooze. We have also the bones of 
mammals and fish mixed up in different kinds of deposits. These, 
as well as animal and vegetable tissues, generally are a source of 
phosphates, fluorides, some oxide of iron, and possibly of other inor- 
ganic material. 
Sir Wyville Thomson, early in the cruise, suggested that much 
of the inorganic material in deposits is derived from the source to 
which I have just alluded. Our subsequent observations have, I 
think, shown that originally Sir Wyville gave too much importance 
to this as a source of the materials in our deep deposits. 
Second . — We have the dust of deserts, which is carried great 
distances by the winds, and which, falling upon the ocean, sinks to 
the bottom and adds to the depositions taking place. In the trade 
wind regions of the North Atlantic we have a very red-coloured 
clay, in deep water, which is largely made up of dust from the 
Sahara. Such dust frequently falls in this region as what is called 
blood-rain. 
Third . — We have the loose volcanic materials, which have been 
shown to be universally distributed as floating pumice, or as ashes 
carried by the wind. 
This short review shows that the clay in shore deposits is chiefly 
derived from river and coast detritus. As we pass beyond about 
one hundred and fifty miles from the shores of a continent the 
character of the clayey matter changes. It loses its usual blue 
colour, and becomes reddish or brown, and particles of mica and 
rounded pieces of quartz give place to pumice, crystals of sanidin, 
augite, olivine, &c. All this goes, I think, to show that in depo- 
sits far from land the clay is chiefly derived from volcanic debris, 
though in the region of the North Atlantic trade winds much of it 
may be derived from the feldspar in the dust of the Sahara. 
The pumice which floats about on the surface of the sea must 
be continually weathering, and the clay which results and the 
crystals which it contains will fall to the bottom, mingling with 
the deposit which is in course of formation. In our purest globige- 
rina ooze this clay and these crystals are present. If a few of the 
