1909-10.] Composition and Character of Oceanic Red Clay. 189 
The “insoluble” residues, which (excepting No. 7, where black specks of 
ferruginous mineral were plainly visible) consisted of fine white powders 
sometimes rendered bluish by organic matter, were separately analysed. 
The analyses were carried out with as much refinement as was considered 
adequate for so variable a material. Double precipitations were not made 
except in the determination of sesquioxides. Manganese was precipitated 
by the bromine method, and a correction (not to be neglected) for the 
manganese escaping into the Ca and Mg precipitates was made by dissolving 
these latter, after weighing, in dilute nitric and sulphuric acids respectively, 
oxidising with sodium bismuthate, and titrating the permanganate formed. 
Silica was invariably corrected by evaporation with hydrofluoric and 
sulphuric acids. Titanic and phosphoric acids were not determined, 
and may be supposed to lie hidden in the item A1 2 0 3 . Barium eventuated 
principally in the residue as sulphate, but was apt also to go partially into 
the “ soluble ” portion ; in both portions it was brought down together with 
silica and duly allowed for ; the BaO reported below, however, though 
classed in the “ soluble ” category, refers to total baryta determined on a 
separate batch of material. Alkalies in the anhydrous residue were de- 
termined by the Berzelius method ; for “ soluble ” alkalies a separate gram 
of material was treated with sulphuric followed by hydrochloric acid, the 
filtrate was evaporated to expulsion of free sulphuric acid, and the sub- 
sequent procedure was as usual. All suspicious or unexpected results were 
checked by at least one redetermination. 
The analytical figures are displayed in Table A, arranged in what seems 
to be the most rational manner. The item “ residue ” represents the total 
percentage of anhydrous silicates, and its composition is stated independ- 
ently at the foot of each main analysis. In all cases the main analyses are 
referred to material dried in an air-bath at 110°. It is true that the degree 
of hydration of a clayey mineral depends not only on the temperature, but 
also on the tension of water- vapour in the surrounding atmosphere, so that 
in theory the material ought to be neither air-dried nor oven-dried, but 
brought into equilibrium (time required, several months) with air of a 
definite moistness at a definite temperature. However, drying at 110° was 
found to give sufficient constancy for present purposes. 
It is seen that the proportion of anhydrous silicates in Red Clays 
fluctuates somewhat : it lies mostly in the neighbourhood of 30 per cent., 
but is exceptionally low in South Atlantic (No. 3) and South Pacific 
(Nos. 8 and 9) specimens and exceptionally high in Nos. 7 and 11. A 
glance at the detailed analyses of these residues shows that they consist 
in general of highly acid alkali-aluminium silicates with vanishing 
