EEPORT ON THE DEEP-SEA DEPOSITS. 
31 
from among several thousands, but not to give us the slightest indication of the ocean or 
depth from which the specimen was obtained. After examination, we have then marked 
regions on the chart in which we believed it was collected, stating at the same time the 
probable depth. In the great majority of cases, in about nine out of ten trials, the 
position could be stated within a few hundred miles and the depth within a few hundred 
fathoms. A few of the considerations on which we relied in making such determinations 
may fitly terminate this chapter on the methods of study. 
The presence of large numbers of Pteropod and Heteropod shells indicates tropical or 
subtropical regions, and relatively shallow depths. Abundance of the shells of pelagic 
Foraminifera indicates the same regions, but when found without the shells of pelagic 
Molluscs they indicate a greater depth than when these latter are present. A very few 
fragments of these pelagic organisms, and consequently a low percentage of carbonate of 
lime, with abundance of the red and yellow oxides of iron and the black oxide of 
manganese, point out again still greater depths in the tropical regions. The presence 
or absence, and the size, of Ehabdoliths, Coccoliths, and Coccospheres give important 
indications as to latitude and depth — the first predominating in tropical regions, 
the two latter being better developed in temperate regions, and all disappear from the 
deposits as the polar waters are approached. Take again the remains of those lime- 
secreting organisms that habitually five at the bottom of the sea, such as Foraminifera, 
Polyzoa, Ostracodes, Molluscs, Corals, Annelids, and Algae. The greater or less abund- 
ance of these in a deposit give most useful indications as to the depth and the distance 
from land at which the specimen was collected. These organisms are as a whole more 
abundant and better developed in shallow water, and in both these respects a change is 
observed in their fragmentary remains in greater depths and at a greater distance from 
land. Some species, however, denote, when present in abundance, ranges of depth. The 
greater or less abundance of some of the remains of the pelagic species give indications 
as to longitude ; for instance, some pelagic species of Foraminifera are much more indi- 
cative of an Atlantic deposit than of a deposit from a similar latitude and depth in the 
Pacific. 
In the same way the remains of siliceous organisms may furnish information as to 
depth and locality. The frustules of the large Diatom Ethmodiscus, Castracane, is quite 
characteristic of some of the deepest tropical Eed Clays and Eadiolarian Oozes far from 
land ; it is quite absent in temperate and polar regions. A typical Diatom Ooze is only 
found in the neighbourhood of the great Southern Ocean surrounding the Antarctic 
continent, although some deposits that might be called Diatom Oozes are found in the 
most northern parts of the Pacific. A typical Eadiolarian Ooze is limited to certain of 
the deeper tropical and subtropical portions of the Indian and Pacific Oceans. 
When we consider the mineral particles, they too testify as to the conditions under 
which the deposit was formed. Typical glauconite and glauconitic casts appear to be 
