REPORT ON THE DEEP-SEA DEPOSITS. 
395 
carbonate of lime of the shell is often observed. According to the structure or 
thickness of the calcareous partitions, this substitution is more or less advanced ; some- 
times the structure of the shell is more or less well preserved, but frequently it is 
quite effaced. When the phosphate has invaded the interior of the Foraminifera, and 
the calcareous partitions have not been touched by the pseudomorphism, the sections 
of the partitions stand out pure and colourless, showing that the infiltrated matter 
has penetrated by the foramina ; at other times the shell assumes a yellow appearance, 
showing the first step towards phosphatisation (see PI. XX. fig. 3). When the filling- 
up of a Foraminifer, for example, and the pseudomorphism of its shell are complete, the 
phosphate, attracted around this little centre, continues to be added at the surface, and 
thus a phosphatic granule is formed whose external appearance no longer recalls that of 
the organism arcmnd which the phosphate has grouped itself. This observation is not 
without importance in the interpretation of the origin of the phosphatic grains. The 
study of microscopic sections of these Foraminifera confirms a fact often brought out in 
descriptions of phosphatic fossils, viz., that the infiltration of the phosphate has a direct 
relation, so to speak, to the fineness of the openings by which this matter must be 
introduced. Thus a large number of the Foraminifera may be seen to be filled with 
phosphate, while very often in the fundamental mass at the mouth of the shell there are 
points where the phosphatisation has not taken place, being still dotted with particles of 
carbonate of lime showing clearly the optic phenomena of that mineral. It may be 
said that when the sections of shells of Foraminifera no longer exhibit the black cross 
between crossed nicols, they are transformed into phosphate. 
Sometimes the phosphate of lime takes on an ochreous or brownish tint, showing 
that it is mixed, as already indicated, with manganiferous and ferruginous matters 
— its usual accompaniments in marine sediments — or with organic matters. Although 
the yellowish tint is characteristic, it may also be replaced by a greenish coloration, 
when it is sometimes difficult to distinguish phosphate of lime from glauconite. 
Means of distinction, however, may be found in the concretionary forms of the 
phosphate,' giving it a zonary structure, even recalling by its capricious lines an 
osseous structure at first sight, while, on the other hand, the aggregate polarisation 
of glauconite affords a means of differentiation, which, after a little practice, may 
be applied with certainty. In doubtful cases it is well to have recourse to micro- 
chemical reaction, when, with the aid of molybdate of ammonia, the question may be 
decided in a sure and rapid manner. 
Distribution and Mineral Associations. — Having described in detail these phosphatic 
concretions, we may now consider the conditions under which they have been formed. 
It has already been stated that these nodules were dredged by the Challenger at 
Stations 141, 142, and 143, after leaving the Cape of Good Hope for the southern 
cruise. The two former are situated on the Agulhas Bank, on the submarine edge 
