JoLY — 8o7ne Experiments on Denudation. 29 



But they suggest forcibly that the final results in natiu-e (or in the 

 experiments), as regards bringing the rock materials into solution, 

 represent but a part of the total reaction upon the rock. In other 

 words, the amount of decomposition actually effected is indicated only 

 by the liberation in solution of certain of the constituents. This fact — 

 which could be instanced by many well-known phenomena of rock- 

 weathering — involves a conservative effect of great importance in 

 nature, and which must also be borne in mind in considering any such 

 experiments as the present ones, effected on fresh material. The pro- 

 cess of leaching out soluble constituents, and leaving insoluble ones, or 

 those of secondary formation, behind, must lead to a rapid diminution 

 of the surface activity of the solid. 



"With the conspicuous exception of the orthoclase the silica obtained 

 in the salt water solutions is either about eijualled or actually largely 

 excelled in amount in the case of the fresh-water solutions — as in the 

 experiments on basalt and obsidian. 



The obsidian, it will be observed, proved to both solvents the most 

 resistant of the materials dealt with. Daubree records among his 

 results that this same substance offered remarkable resistance to 

 attack.^ The final solutions in this case, he records, showed scarcely 

 any alkaline reaction. 



A conspicuous featui'e of the results is the much greater quantities 

 of lime dissolved in the salt-water solutions. According to Bischof 

 this might be explained, as we have seen, as the result of the 

 secondary reaction attending the liberation of alkaline silicates. The 

 result, which is more especially conspicuous in the case of the basalt 

 (^both in fresh and salt water), is in keeping with the well-known 

 deposition of carbonate of calcium in some basic igneous rocks 

 undergoing decay. Alumina in solution was only detected in the 

 salt solutions, but in every case in minute quantity. The almost 

 complete absence of ii'on in the solutions is remarkable, the delicate 

 test by sulphocyanide of ammonium revealing no more in any instance 

 than a trace. 



Looking at the figui'es at the foot of the columns of Tables i. or ii., 

 we observe that in every case the total amount removed in solution by 

 the salt water exceeds considerably what is removed by the fresh water. 

 If the alkalies (and magnesia in some cases) taken up by the sea water 

 were added, the preponderance would be still greater. As it stands the 



1 loc. cit., p. 275. 



