390 
THE VOYAGE OF H.M.S. CHALLENGER. 
aiul minerals, which make up a large part of the muddy matters settling on the bottom 
beyond the mud-line around continental shores, would readily yield under the action of 
sea-water the chemical elements that are deposited in the form of glauconite in the 
chambers of Foraminifera and other calcareous organisms. 
Other Casts of Foraminifera . — In the Tables of Chapter II. it will be observed that 
imperfect casts of Foraminifera are very frequently recorded in the residues after the 
removal of the Foraminifera by dilute acid. In the great majority of instances these are 
of a reddish or brownish colour, and appear to be formed of a substance which lined with 
a thin coating the internal chambers of the shells. They hold together with some tenacity 
in water, but immediately collapse when dried upon a platinum foil, and sometimes they 
become black or burn, leaving a small reddish residue. At other times phosphates can be 
detected in these imperfect casts. As a general rule, a few red-coloured more or less im- 
perfect casts of the internal chambers of Foraminifera may be found in nearly all 
calcareous deposits, but internal casts are only present in abundance in those regions 
where glauconite is in process of formation, and have been fully referred to above. 
At Station 176 in the South Pacific, large numbers of peculiar casts were observed in 
a Globigerina Ooze from 1450 fathoms, which are markedly different from the glauconitic 
casts. The Foraminifera, in the deposit from this station, presented a very mottled aspect 
under the microscope, some of them being white or rose-coloured, as is usually the case in 
a Globigerina Ooze, while others were brown or black, from a deposit of the peroxides of 
iron and manganese on their outer surfaces. When a section is made through these black 
or brown-coloured specimens, three zones can be distinguished : at the centre an internal 
cast of the shell, then the white carbonate of lime shell itself, and outside this an external 
cast of the same nature and aspect as the internal one, to which it is connected by little 
pillars filling up the foramina of the shell (see PI. XL fig. 1). When the carbonate of 
lime Ls removed from such specimens, it is seen that the external cast is in general not 
thicker than the hollow space left by the removal of the shell, and that this external cast 
can be partially separated from the internal one by the use of a little force. The general 
appearance of these external and internal casts is represented in PI. XXIV. fig. 4, and 
it will be observed that they differ, owing to the j^resence of the external casts united 
to internal casts by little pillars, from the specimens represented in the other figures 
on the .same plate, w’here we ob.scrve only internal glauconitic casts. The red-coloured 
casts from this station offer considerable resistance to the action of acids and mechanical 
effort, which seems to show at once that we are not dealing with a cast made up merely 
by a simple filling of the shell with fine mud or clay. The red casts, when examined in 
thin sections by transmitted light, are yellow or brown, scattered over by a fine granu- 
lation, which is not affected between crossed nicols. When treated with warm hydro- 
chloric acid we obtain, by elimination of the iron, colourless globules that appear to have 
almost completely resisted the action of the acid, and are in all likelihood composed 
