28 Proceedings of the Royal Irish Academy. 



evidence on the broad question at issue I have not found. Dana con- 

 sidered that basalt rocks were protected by sea-water, either where 

 quite covered or merely washed with spray, relatively to the same 

 rock exposed to the alternate wetting and drying of sub-aereal actions. 

 Merrill/ commenting on this, remarks justly that erosive actions, in 

 such cases, preserve a deceptive appearance of freshness to the rock. 

 He, however, thinks that no exception can be taken to Dana's remarks 

 regarding rocks wholly immersed. 



Grustav Bischof in his well-known " Chemical and Physical Geology" 

 has advanced reasons fi'om the chemical point of view for believing 

 that the alkaline silicates of felspars, &c., will experience more active 

 dissolution in water containing dissolved salts of calcium and mag- 

 nesium. This view is based on the fact that alkaline silicates are 

 decomposed in presence of the sulphates and chlorides of calcium and 

 magnesium, the sparingly soluble earthy silicates being precipitated.'^ 



If this applies to the naturally occurring crystallized silicates, in 

 which alumina forms part of the molecule, and which are, compara- 

 tively speaking, insoluble bodies, sea-water, containing MgSOi, CaS04, 

 and MgCl2, in abundance, should accelerate the decomposition of 

 felspars. 



The results of the reaction with the alkaline silicates appear, 

 according to Bischof, to be the formation of the silicates of lime and 

 magnesium and the sulphates and chlorides of the alkalies. The 

 latter will, of course, be dissolved. The silicate of lime will again be 

 decomposed if carbonic acid is present, silica separating out and preci- 

 pitating, and carbonate of calcium being formed. The silicate of 

 magnesia will, however, not be decomposed by carbonic acid. 



According to these reactions, wherever sea-water acts upon silicates 

 containing alkaline silicates, and is, as in the experiments, freely 

 exposed to the CO2 of the atmosphere, decomposition will be accele- 

 rated, silicate of magnesia being precipitated, bicarbonate of calcium 

 formed and retained in solution (or precipitated if the amount of CO2 

 is deficient), silica precipitated, and soluble chlorides and sulphates of 

 the alkalies formed. These reactions would alone not serve to explain 

 the presence of the comparatively large amount of silica in solution 

 revealed by the reaction with orthoclase (Ex. m.), unless soluble 

 alkaline silicates remain in solution, or a hydrosol of silica is formed. 



* Loc, cit., p. 253. 



- " Geologie Experimentale," vol. i., p. 12. 



