GEOLOGY OF BOTTOM OF SEAS—DE LAUNAY. 343 
the top. Some have believed in a modification in the conditions of 
deposition; I would rather believe in a later alteration. At the same 
time, however, as at the surface, the proportion of magnesia increases 
because the bicarbonate of magnesia and of lime is less soluble than 
the carbonate of lime: there is dolomitization. 
On the other hand, there is produced at centers of attraction, ex- 
actly as at our horizon, a concentration of silica and other accessory 
bodies to which I shall presently revert, such as iron, manganese, 
and phosphate of lime. It is very probable that many similar con- 
centrations observed in our geologic strata date from the epoch 
when the sediments in question were still under the sea, although 
the phenomena may undoubtedly have been continued and accen- 
tuated after emergence. Among the phenomena of dissolution one 
may still observe that on the great marine bottoms where the sharks’ 
teeth and the tympanum drums of whales are at times rather abun- 
dant, all other parts of their skeletons have been dissolved; gen- 
erally all that was phosphate of lime has been eliminated, the calca- 
reous parts having the greater resistance. 
Once dissolved, the substances tend to recrystallize. Calcite, for 
example, will refill all the empty spaces, notably those left by the 
dissolution of the skeleton or of the shell, which is thus replaced by 
a substituted shell with crystallographic orientation. 
Likewise in malm rock, the silica arises from the spicules of the 
sponges and is reprecipitated in the form of silica globules for ex- 
ample, in the interior of the foraminifera, etc. 
One of the most important of the oceanographic formations which 
must be connected with diagenesis is that of the red clays with de- 
posits of manganese. 
Red clay was found for the first time by the Challenger at a depth 
of 5,000 meters, and Wyville Thomson considered it a residue of the 
globigerina ooze. The accepted theory is otherwise, and according 
to Murray this red clay is ordinarily attributed to the decomposition 
of various rocks, especially the volcanic rocks which form the marine 
bottom. 
Perhaps this would be the place to review, in a way, the explana- 
tion of Thomson and to say that in the very great depths everything 
is transformed into red clay because everything calcareous is dis- 
solved before reaching there. Thus at the surface of continents we 
see red clays of slightly varied compositions,! with more or less 
silica associated with the alumina, produced as well on the calcareous 
plateaus, in the so-called pockets of siliceous clay, as on the serpen- 
tines of New Caledonia or on the ancient plateaus of Madagascar. 
1 See Gites métal., vol. 1, p. 197, on lateritization. 
