REPOET ON THE DEEP-SEA DEPOSITS. 
169 
formation of the deeper layer. It is clearly illustrated in this section between Api and 
Eaine Islands, that all the other conditions remaining the same or nearly so, the quantity 
of carbonate of lime found in a deposit is less the greater the depth. It is believed that 
this basin below 1300 fathoms is probably cut off from the colder water farther south, and, 
indeed, from general oceanic circulation, below that depth, in this respect approaching to 
the condition of enclosed seas. In all such basins the surface shells appear to be removed 
from the deposits at lesser depths than in areas where there is no interruption to free 
communication arising from the existence of submarine barriers. 
The mineral particles in these deposits consisted chiefly of angular fragments of 
volcanic rocks and minerals, all of small size except the pieces of pumice which were 
numerous in all the dredgings. There were many manganese particles, and, at the 
sounding in 1400 fathoms, some of the Foraminifera shells were filled with the peroxide, 
so that a complete internal cast of the shell was left after treatment with dilute acid. 
The deposit in 130 fathoms, off Api Island, was a Volcanic Sand containing 13 per cent, 
of carbonate of lime. 
Off Raine Island . — The soundings and dredgings in 135, 150, and 155 fathoms (see 
Chart 27) showed that the deposit was a Coral Sand, com^^osed of white and brownish 
coloured fragments of Corals, Molluscs, and Foraminifera shells, with a considerable 
admixture of calcareous Algae. Mr. H. B, Brady found in this deposit a larger number 
of species of Foraminifera than in any other taken during the cruise. Many of the 
shells were probably washed from the shallower water of the adjoining reefs. The deposit 
contained 87 per cent, of carbonate of lime, and it was estimated that more than one-half 
of this consisted of pelagic Molluscs and pelagic Foraminifera. The mineral particles in 
the deposit consisted of fragments of quartz, felspars, mica, augite, and olivine, and were 
estimated at 4 per cent. 
By treating this deposit with dilute acid, casts of the Foraminifera shells are obtained, 
the majority of which are of a brick red colour, although a few are of a yellowish, or even 
greenish, tinge (see PI. XXIV. fig. 3). They are not so compact or well marked in outline 
as the white and pale straw-coloured casts usually met with in glauconitic muds, and have 
very frequently a porous aspect, from the removal of the carbonate of lime which has, in 
many instances, been associated with the red material forming the casts. If some of the 
Foraminifera be treated with dilute acid, the action stopped after it has continued for some 
time, and the substance dried and examined by reflected light, a number of casts of the organ- 
isms are seen in carbonate of lime looking quite like milky quartz. If, however, the action 
be continued, it is seen that they are composed of carbonate of lime as they entirely disappear, 
leaving a small residue of a reddish colour, or very areolar casts of the shells in the same 
red substance. Examined in thin sections, it is observed that the shells are filled with a 
red, yellowish, or greenish matter, frequently extending into the foramina. The shell is 
at once distinguished from the cast by its structure, transparency, and optical properties. 
(deep-sea deposits chall. exp. — 1890.) 22 
