344 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
The red clay of the seas is formed essentially of brick red silicate 
of alumina, with a tint of chocolate brown, or even blackish, due to 
the more or less marked development of manganese. 
This development of manganese often in large nodules, constitutes 
a comparison besides with surface formations where I have shown 
how characteristic and abundant it is! The manganese of the red 
clays, which has too exclusively been connected with volcanic rocks, 
really comes from all kinds of eroded rocks in which the feeblest 
manganiferous traces occur in the remarkable process of its concen- 
tration. 
The hydrated silicate of alumina which is the characteristic ele- 
ment of these red clays is accompanied by zeolites, by small frag- 
ments of pumice or by volcanic crystals (in the Pacific with millions 
of sharks’ teeth, the tympanum bones of whales, etc.). It would be 
interesting to point out the quantitative differences that these various 
clays present which must pertain to their origin. On the average, in 
the analyses of the Challenger the quantity of iron was more than 
that of alumina. There is often a proportion of silica too great for 
the theoretic composition of silicate of alumina; the silica, due no 
doubt to the precipitation of siliceous organisms, not yet being trans- 
formed. Likewise, the proportion of carbonate of lime always re- 
mains very great, up to 23 per cent in a Challenger specimen, from 
a depth of 4,207 meters. Carbonate of magnesia reaches 3.24 per 
cent, and results, no doubt, from a concentration similar to that pro- 
duced on the coral reefs. 
(¢) FORMATION OF PHOSPHATES AND FERRUGINOUS SEDIMENTS. 
We come now to study the ordinary and common deposits which 
constitute the great mass of sediments. There are, however, some 
rarer deposits more exceptional, which may be of practical interest 
to us. They are those composed of utilizable accumulations of 
mineral wealth, the relative commercial value of which depends on 
whether its elements have been brought together there in an abnor- 
mal manner. The sea, which I have elsewhere called the universal 
drain, holds some traces of all chemical bodies, including gold, which 
is found there in quantities not at all negligible. This or that circum- 
stance may cause the concentration of one of these elements and 
from it forma true ore. I shall be content to examine, as examples, 
two bodies, phosphate of lime and iron, having already said a word 
about manganese. 
Let us first take the phosphates. If a direct examination be made 
of the phosphates of our geologic strata without first having recourse 
to a comparison with oceanography one finds that phosphate of lime, 
very abundantly, one might say very generally, scattered through 
1 Gites métal., vol. 1, p. 200, and vol. 2, pp. 530-536. 
