184 
Proceedings of the Koval Society of Edinburgh. [Sess. 
Here and there Red Clay borders upon and merges into deposits which 
are not of a calcareous nature. In the middle of the Pacific there is an 
approximately equatorial belt of Red Clay mixed with biologically pre- 
cipitated silica, to which deposit the specific name “ Radiolarian Ooze ” is 
applied. Along the Arctic and Antarctic circles Red Clay borders upon 
another kind of biological siliceous deposit, viz. Diatom Ooze, whilst around 
the Pacific coast-lines and off the Newfoundland bank direct transitions 
from terrigenous deposits (Blue Muds) to Red Clay have been observed. 
As it lies at the bottom of the ocean, Red Clay is macroscopically far 
from homogeneous. Objects such as manganese nodules, otoliths, sharks’ 
teeth, lumps of pumice and of palagonitic tuff, etc., are disseminated in it. 
Among microscopic admixtures mineral fragments are never absent, whilst 
the following may or may not be present : — calcareous and siliceous products 
of organised life, phillipsite crystals (formed in situ and limited to the South 
Pacific and Indian Oceans), and cosmic particles. All these have an interest 
of their own which is outside the scope of the present considerations, and in 
point of quantity they play no part whatever in comparison with the matrix 
itself. It is this latter to which the term “ Red Clay ” is here applied. 
Now all Red Clays are composed of two chemically distinct ingredients, 
viz. hydrated amorphous silicates of an argillaceous character and finely 
divided anhydrous silicates, partly vitreous and partly crystalline, which 
represent what is left of the mother-substances of the former. The first- 
named component is that which imparts to the Red Clay its characteristic 
consistency, and it does not differ in general physical properties from the 
essential constituent of terrestrial clay. That clay-substance is of a colloidal 
nature had been recognised when or before oceanic Red Clay was dis- 
covered,* and has been amply confirmed in the course of the great advance 
of our knowledge of colloids made during the last decade or two.j* A 
corollary from this fact which was formerly overlooked is that a hard and 
fast chemical composition is not to be attributed to clay-substance. It so 
happens that kaolin, which used to be regarded (erroneously, because it 
often consists largely of fine crystalline matter) as ideally pure clay, answers 
approximately to A1 2 0 3 . 2Si0 2 . 2H 2 0, and a compound of this formula was 
at first assumed to be the essential ingredient of Red Clay. This view, 
however, can no longer be considered unassailable. As a matter of experi- 
mental fact there is far more silica in Red Clay than corresponds to the 
“ ideal clay ” formula. 
* Challenger Reports, “Deep-Sea Deposits,” 1891, pp. 190, 340. 
t For an account of the colloidal characteristics of clay see Rohland, Abegg’s Handb. d. 
anorg. Chem., in. 1 , pp. 90-102, 1906. 
