30 Proceedings of the Royal Irish Academy. 



sea-water dissolves from twice (obsidian) to fourteen times (orthoclase) 

 the mass dissolved by fresh water. 



The main question at issue is undoubtedly answered by these 

 experiments, incomplete though they be. They show, indeed, that 

 under the conditions of experiment : — moderate temperature ; fresh 

 material ; abundant aeration ; active circulation ; absence of attrition : 

 marine solvent denudation exceeds in activity fresh-water denudation 

 in the case oihasalt not less than three times ; in the case of hornblende 

 not less than eight times ; in the case of obsidian over four times ; and 

 in the case of orthoclase not less than fourteen times. In short, taking 

 alkalies into account and some MgO (as we have seen there is some 

 reason to believe MgO will not enter largely into solution in the case 

 of sea water), the preponderance ranges from about four times (basalt) 

 to seventeen or eighteen times (orthoclase). "With the lapse of time, 

 as the surface of the solids become exhausted of the more soluble 

 constituents, a convergence and approximation of the two rates will 

 probably occur. 



It is interesting to place the figures applying to fresh- water solvent 

 denudation side by side with estimates which have been based on river- 

 water analyses. 



Mr. T. Mellard Eeade has estimated that solvent denudation in 

 England and Wales is lowering the surface of the land at the rate of 

 one centimetre in 430 years. This represents the removal of about 

 60 grammes per square metre per annum. The Mississippi, drawing 

 its supplies from areas exposed to wide climatic extremes and from 

 every variety of rock and soil, is lowering its basin at the rate of one 

 centimetre in 833 years, which represents the removal of about 

 30 grammes per square metre per annum. Comparing these figures 

 with the experimental figures, we see that even a brisk continuous 

 washing of fresh rock-surface having the superficial area of the denuded 

 region would not be competent to supply more than a small percentage 

 of these amounts. The mean of the figures at foot of the columns apply- 

 ing to fresh -water denudation in Table ir. is just 0-08 grammes 

 removed per square metre per annum. This is 0-15 per cent, of the 

 amount estimated by Mr. Mellard Eeade, and 0-3 per cent, of the 

 amount removed per square metre per annum by the Mississippi. 

 Herein we see the influence primarily of the great sui^face areas 

 exposed in the soils (as much as 500 square metres in the litre), 

 as well as the solvent influence of the acids originating in vege- 

 tation, the more rapid solution of the calcareous rocks, effects of 

 alternate wetting and drying, frost and sunshine. 



