198 
Proceedings of the Royal Society of Edinburgh. [Sess. 
agrees with these observations that in Red Clays anhydrous crystalline 
silicates often occur in a state of great freshness, whereas pumice fragments 
are invariably much corroded and basic glass is generally (except when 
preserved in manganese nodules) wholly palagonitised. 
An interesting feature of deep-sea weathering is that it takes place 
under conditions which admit of finality. In the Red Clay areas we have 
a temperature of l°-3°, a pressure of 400-600 atm., and a uniform 
medium (sea-water) which have scarcely changed for millions of years. 
As the result, we find a degradation-product of much the same composition 
all over the globe, and we note that it is a more acid silicate than the 
corresponding continental material. This acidity cannot be due to non- 
elimination of silica, since the mother-substances themselves are far more 
acid. Clearly silica can escape into the hydrosphere just as well as 
alkalies and alkaline earths.* On the whole, there seems to be something 
approximating to genuine equilibrium between the argillaceous portion of 
Red Clay and sea-water, modified only by the inability of the two 
sesquioxides to transfer themselves freely between deposit and water. If 
there were no iron, then, since silica can pass in and out of solution, a 
constant ratio of A1 2 0 3 to Si0 2 would be striven for in equilibrium ; but, 
as things are, so much iron must remain in the deposit as happens to be on 
the spot. Hence the variations illustrated by Nos. 9 and 10. 
In subaerial weathering, per contra, the conditions are of the widest 
conceivable variety, and are such that a state of chemical equilibrium is 
rarely, if ever, reached. Thus we find that in temperate climates pro- 
ducts having roughly one molecule of alumina to two of silica are the rule, 
whereas in the tropics the tendency is toward the highly basic laterites, 
culminating in almost non-siliceous bauxites. 
On land, circumstances occasionally arise in which constituents of the col- 
loidal clay-agglutinate can combine to true chemical individuals. Such are 
the crystalline minerals kaolinite, Al 2 Si 2 0 7 . 2H 2 0, hydrargillite, A1 2 0 3 . 3H 2 0, 
and perhaps nontronite, Fe 2 Si 2 0 7 . 2H 2 0, not to mention micas, chlorites, stauro- 
lites, felspars, etc., of secondary origin. No such formations are known in 
deep-sea deposits ; at any rate, no crystalline matter of secondary growth, 
derived from colloidal materials, can be detected microscopically. On the 
other hand, there are conditions in parts of the South Pacific which favour 
the deposition of well-crystallised zeolites (sodio-potassic phillipsite) in the 
midst of Red Clay deposits, a phenomenon which has no parallel in con- 
tinental clays. In the reaction by which these crystals were formed 
* Silica cannot, however, accumulate in solution, but is returned to the bottom by 
biological agencies as diatom or radiolarian ooze. 
