196 
Proceedings of the Royal Society of Edinburgh. [Sess. 
respect to the amorphous products of subaerial weathering. In so far as 
they are amorphous and colloidal, these hydrous silicates are to be regarded 
not as definite chemical compounds or as mixtures of such, but as agglutin- 
ates of colloidal silica, alumina, etc., in inconstant proportions. Just as 
clay itself attaches water of hydration not in a series of stoichiometrical 
proportions (like, for instance, the hydrates of MgS0 4 or Na 2 C0 3 ) but 
in continuously variable proportions depending on such conditions as 
temperature, pressure, and medium, so the chief constituents of clay 
form an agglutinate in proportions similarly governed by active masses 
and other extraneous factors. What the affinity is which binds the 
constituents together we do not know, but it is certainly not exclusively 
chemical. 
The irregular ratios which obtain when alumina combines with silica to 
form an amorphous hydrated solid are not only illustrated in nature, but 
are also well brought out by synthetic experiments such as those of Lemberg* 
and of Stremme.f The latter investigator, who analysed not only artificial 
alumino-silicic precipitates, but also minerals of the allophane, halloysite, 
and montmorillonite groups, comes to the conclusion that all these bodies 
are merely mechanical mixtures of colloidal hydrated alumina and silica. 
The same view with respect to clay has also been expressed by Rohland.J 
There are two considerations, however, which may be urged against the 
suggestion that clays are nothing more than mixtures. In the first place, 
there is the resistance offered by clays to acid attack ; if they were mixtures, 
extraction of all the argillaceous alumina by dilute acid ought to be a 
simple matter, whereas in reality no less drastic a reagent than hot concen- 
trated sulphuric acid is required for the complete decomposition of clay. In 
the second place, it cannot be overlooked that a certain hazy constancy of 
the ratio between silica and alumina prevails in both submarine and 
continental clays, whereby a tendency on the part of these constituents to 
combine rather than exist independently side by side is indicated. Thus 
acid minerals ( e.g . orthoclase, in which A1 2 0 3 ; Si0 2 = l : 6) weather to clays 
which have for the most part ratios ranging from 1 : 2 to 1 : 3 ; now, if the 
latter were mixtures, it is hard to understand why the same agencies which 
have removed so much silica should not also remove the remainder and 
leave bauxite instead of clay. Degradation-products intermediate between 
clay and bauxite do indeed occur, but they are comparatively uncommon 
minerals and are therefore doubtless produced by exceptional means. 
* Zeitschr. deutsch. geol. Ges ., xxviii. p. 519, 1896. 
t Centralbl.f. Min., 1908, pp. 622, 661. 
X Loc. cit. 
