REPORT ON THE DEEP-SEA DEPOSITS. 
397 
of temperature, and may in some instances form a considerable layer of decomposing 
matter on tbe bottom of the ocean. It is also well known that large numbers of pelagic 
creatures are in like manner destroyed where there is a mixture of waters of different 
salinities, for instance, where polar and equatorial currents mingle, or where large 
quantities of fresh water are thrown into the ocean from floods in great rivers.^ By 
taking account of phenomena such as these, which would result in the destruction of 
large numbers of pelagic animals at one time, thus covering the deposit in process of 
formation with a vast layer consisting of the dead bodies of marine animals, it is believed 
that the origin of the thin bands of phosphatic nodules, so frequent in geological forma- 
tions, may be accounted for. 
The phosphatic nodules observed in existing deposits belong then to the coast zone. 
They may be found in all terrigenous deposits, and also along the edge of the abyssal 
zone in deposits of a pelagic type, which, however, from their nearness to land, still 
contain terrigenous elements. The resemblance of these deposits to those of geological 
formations containing phosphorites in greatest abundance — the greensands, glauconitic 
chalks, and pure chalks — is so evident that it is unnecessary to insist on it. The mode 
of formation of the one must have been almost identical with that of the other, and the 
interpretation of the origin of the phosphatic concretions of existing seas should be 
equally applicable to those of the Cretaceous and Tertiary formations, for example.^ 
Keference has already been made to the analogies between the phosphatic nodules of 
modern sediments and those of a great number of nodular phosphates of the chalk and 
greensand formations, so much so that it might even be asked whether the concretions 
described in this chapter might not be derived from ancient formations cropping out at 
the bottom of the sea. This doubt is at once removed when account is taken of the 
fact, already pointed out in treating of the microstructure of these concretions, that they 
contain, cemented and enclosed by phosphates, the remains of organisms and mineral 
particles identical with those constituting the actual sediments in which the concretions 
are found. These phosphatic concretions must therefore be regarded as having been 
formed in situ. 
Mode of Formation . — If we ask whence the phosphate of lime found in these nodules 
is immediately derived, we may set aside in the first place the hypothesis of a direct 
derivation from the interior of the globe, for although it is evident that in certain cases 
a small percentage of phosphate of lime in deep-sea muds might be attributed to apatite 
coming from volcanic rocks, still even at the highest estimate the amount of phosphate of 
lime coming from this source must be very subordinate relative to that derived, for in- 
stance, from organic remains. Nor is there any reason in the conditions under which 
they have been formed for supposing that the phosphate of lime could have been derived 
from submarine springs. Again, we find nothing in the surroundings to induce us to 
1 Murray, Scot. Geogr. Mag., vol. vi. pp. 481, 482, 1890. ^ Murray, loc. cit., pp. 464, 465. 
