I90 DEPTHS OF THE OCEAN 



the fixation of potassium must reach formidable dimensions, 

 since the purest Green sands may contain 7 to 8 per cent of 

 K2O. Nevertheless over the whole ocean it is hardly probable 

 that deposition keeps pace with supply, and potassium may be 

 regarded as one of those elements which are slowly concentrating 

 in sea-water. 



The zeolite phillipsite is the only substance produced in 

 well-developed crystalline forms at the bottom of the sea, where 

 it is peculiar to the deepest Red clay regions. Marine phillips- 

 ite is a hydrated calcium-aluminium silicate in which the 

 principal minor bases are potash and soda (4 to 5 per cent each 

 of KoO and NagO), with insignificant amounts of lime and 

 magnesia. Like all zeolites, it must have been deposited out of 

 a solution of its constituents, and it represents an intermediate 

 stage in the degradation of rock-silicates to clay. Why should 

 the process of degradation have been arrested at this stage ? 

 In all probability because solutions containing silica, alumina, 

 and the other elements in just the right proportions were 

 imprisoned in interstices of the Red clay, secure from diffusion, 

 and therefore available for the slow process of crystallisation. 

 It is worthy of note that in point of percentage quantity the 

 minor bases of marine phillipsite differ widely from those of the 

 terrestrial mineral, in which latter calcium plays the chief part. 

 Taking into account the well-known faculty possessed by zeolites 

 of exchanging bases with solutions with which they are in 

 contact we have here (especially in the high percentage of 

 Na^O) an interesting effect of sea-water as a medium in the 

 mineralogical world, comparable with its far-reaching biological 

 effects. Why the crystallographical species phillipsite should 

 be favoured rather than any other zeolite, we cannot in the 

 present state of knowledge imagine. 



The chief submarine concretionary substances are, in 

 descending order of abundance, manganese and iron peroxides, 

 calcium phosphate, calcium carbonate, and barium sulphate. 

 A tendency to assume concretionary forms argues proneness to 

 supersaturation and feebly crystalline habit on the part of the 

 substance concerned. The former property is very characteristic 

 of the peroxides and of calcium phosphate, and is evidently 

 connected with the reluctance to come to equilibrium in solution 

 which so often goes hand in hand with high valencies.' 

 Wherever concretions are found, we must suppose that there 

 has at one time been a layer, or a chronological series of layers, 



^ See Van t'Hoff, Sitztmgsber. K. Akad. IViss. Berlin, vol. xxxiv. p. 658, 1907. 



