•202 
THE VOYAGE OF H.M.S. CHALLENGER. 
The origin of the clayey materials, and the products of the decomposition of the rocks 
and minerals spread over the floor of the ocean, will be discussed in detail later on, but 
from what has been stated above it will be evident that the Eed Clay must be regarded 
as essentially a chemical deposit universally distributed in the ocean basins, but only 
appearing with its typical characters in the greatest depths far from continental land. 
Microscopic examination and chemical analysis have shown that volcanic rocks and 
minerals are to be found everywhere distributed in oceanic formations, and that in many 
regions the most frequent among these belong to the basic series all containing alumina. 
It is known that these very rocks — basalts, andesites, &c. — give by their decomposition 
argillaceous matters, so that we are led to conclude that this clayey deposit is chiefly the 
result of the decomposition in sit to of these substances, as will be shown at greater length 
in Chaptei-s V. and VI. 
Even admitting that chemical decomposition at the bottom of the sea is not more 
active than at the surface of the continents, the rocks and mineral fragments would 
undergo much the same alterations at the bottom of the sea as on the surface of the 
terrestrial masses, where silicates and aluminous rocks are observed to be decomposed 
under the influence of water, and transformed into clayey materials, almost always 
mixed with the other products of decomposition of the rocks and silicates, giving origin 
to clay. These reactions taking place in the greater depths at the bottom of the sea, 
where the waters are not subject to any rapid movements, the products of decomposition 
are not transported to great distances, as is the case on the continents, and therefore, 
as has been already stated, these clays must be impure. The diffusion can not, moreover, 
be very rapid, and the chemical bath can act in a slow and constant manner on the 
solitl materials with which it is in contact. Without entering here into the discussion of 
the question, we seem justified in regarding the greater part of the fine material, as well 
as the zeolites and nodular masses, of the red clay deposits as having been formed in situ 
through chemical action. This result, as will be shown, is not out of harmony with what 
we know of the distribution of the material borne down to the ocean from the land. 
As far as our knowledge at the i)resent time extends, Red Clay would appear to be the 
most extensive of all Marine Deposits, being estimated to cover about 51,500,000 square 
miles, or more than one-fourth of the total area of the globe, as will be seen by an inspection 
of the accompanying map (Chart 1) showing the distribution of Marine Deposits. 
In the Atlantic the area occupied by Red Clay is much less than that occupied by 
Globigerina Ooze, being estimated at about 5,800,000 square miles. It is found in five 
detached areas — two in the North Atlantic, one in the eastern, the other in the western 
|K*rtion of the b:isin, sej)arated by the Dolj)hin Ridge ; three in the South Atlantic, two 
some distance off the South American coast, the other off the African coast, .separated by 
the Cliallenger Ridge running north and south towards the centre of the South Atlantic 
