410 THE DEPTHS OF THE SEA. [CHAP. IX. 
sisted mainly of a compact ‘mortar,’ of a bluish 
colour, passing into a thin—evidently superficial— 
layer, much softer and more creamy in consistence, 
and of a yellowish colour. Under the microscope the 
surface-layer was found to consist chiefly of entire 
shells of Globigerina bulloides (Fig. 2, p. 22), large 
and small, and fragments of such shells mixed 
with a quantity of amorphous calcareous matter in 
fine particles, a little fine sand, and many spicules, 
portions of spicules, and shells of Radiolaria, a few 
spicules of sponges, and a few frustules of diatoms. 
Below the surface-layer the sediment becomes 
eradually more compact, and a slight grey colour, 
due probably to the decomposing organic matter, 
becomes more pronounced, while perfect shells of 
elobigerina almost entirely disappear, fragments be- 
come smaller, and caleareous mud, structureless and 
in a fine state of division, is in greatly preponderat- 
ing proportion. One can have no doubt, on examining 
this sediment, that it is formed in the main by the 
accumulation and disintegration of the shells of — 
elobigerina—the shells fresh, whole, and living in 
the surface-layer of the deposit, and in the lower 
layers dead, and gradually crumbling down by the 
decomposition of their organic cement, and by the 
pressure of the layers above—an animal formation in 
fact being formed very much in the same way as in 
the accumulation of vegetable matter in a peat bog, 
by life and growth above, and death, retarded de- 
composition, and compression beneath. 
In this dredging, as in most others in the bed 
of the Atlantic, there was evidence of a considerable 
quantity of soft gelatinous organic matter, enough 
