TRANSACTIONS OF SECTION C. 731 



tures which in all cases are inferior to the melting points determined on brief 

 observation. It is I'ound that silicates of low percentage of silica show a less 

 depression of melting point than those of high silica percentage, the result beinf 

 that the melting points as newly determined tall into general harmony with the 

 normal order of consolidation. 



The suggestion is oli'ered that the time reqiiired for the crystallised silicates to 

 assume the appearance of fusion is a rough measure of the stability of the 

 crystalline aggregate under conditions of hiuh temperature; for, in fact, it is a 

 measure of the crystalline rigidity opposing the unbalanced molecular surface- 

 force. It is shown that if this be accepted the stability-relations of the silicates with 

 one another may, according to the experiments, vary with temperature. Thus the 

 manner in which the time required to effect fusion varies with the temperature in 

 the cases of leucite and augite would indicate that in a high temperature magma 

 the stability of leucite may be greater than augite, and leucite may crystallise 

 in advance of augite. At lower temperatures augite is the more stable, and would 

 be idiomorphic towards leucite. In a cooling magma every stage of mutual 

 relations may arise. There will be one temperature for such substances at which 

 the stabilities are equal, and at this temperature, phenomena of inteigrowth, such 

 as pegmatitic development, will be favoured. 



Quartz may separate idiomorphically in a high temperature magma, being at 

 high temperatures more stable than most of the silicates, or it may be lelt over as 

 a residual constituent in a low-temperature magma, most of the ferro-magnesian 

 silicates being more stable at low temperatures. Again at low temperatures its 

 stability and that of the felspars approximate, and pegmatitic iutergrowth as a 

 frequent residuum is accordingly to be expected. 



The normal order of cou^olldation follows in general that of the maximum 

 stabilities of the silicates, that is, their stabilities at low temperatures. Abnor- 

 malities of order may be expected to arise more especially in connection with rapid 

 cooling of a magma for long preserved under conditions of high temperature. 



Appearances of magmatic instability, such as resorption, alteration of species, 

 corrosion, &c., are ascribed to changes in the stability relations of the silicates 

 with descending tempei'ature. 



3. On the Geological Age of the Earth as indicated by the Sodium-contents 

 of the Sea. By Prof. J. Joly, D.Sc., F.B.S.—Hee Reports, p. 369, 



4. Some ExperiTiients on Denudation in Fresh and Salt Water, 

 By Prof. J. Joly, D.Sc, F.R.S. 



The question of the relative rates of solution in fresh and salt water of the 

 more important rock materials is considered in this paper, which records the 

 results of experiments on basalt, orthoclase, obsidian, and hornblende. 



In these experiments equal weights of fresh material in the same state of 

 subdivision are exposed to the solvent action of fresh and salt water under like 

 conditions of temperature, aeration, &c. 



In order to secure full aiiratiou, in one set of four duplicate experiments on 

 the substances mentioned, the material is treated in the form of fine powder, and 

 the solvent containing it maintained in motion by a continuous stream of filtered 

 and moistened air escaping in bubbles at the bottom of the containing vessel. The 

 maintained action of atmospheric carbon dioxide and oxygen is thus secured. 

 Alter three months' continuous exposure to these conditions the solvent was 

 removed and analysed. 



In a second foini of experiment, confined to basalt, the fresh material is 

 brought to the grain of a fine sand, and is placed in U-tubes through which the 

 solvent passes alternately in opposite directions, air being freely admitted. This 

 experiment lasted for four months, the circulation of the solvent being maintained 

 for six hours daily. 



