Reports and Proceedings — British Association. 423 



which particular compounds are stable, so that the matter is by no 

 means so simple as might appear from this brief sketch. It is 

 further necessary, in order to bring the process within reach of 

 calculation, to assume that each deposit is removed from contact with 

 the mother liquor after it has crystallised out ; but fortunately this 

 i'8 practically what has happened in the Stassfurt deposits, for each 

 layer is more or less separated from the succeeding one by an 

 intervening layer of clayey material. 



It may be possible even to go a stage further and obtain a clue to 

 the actual temperatures that prevailed, for two minerals, langbeinite 

 and loweite, are absent from the theoretical model made by van 't 

 Hoff to represent what must happen during evaporation at 25° ; and 

 this indicates that while the deposits crystallised the temperature 

 really rose higher than 25°, probably as high as 43° ; in fact, after 

 the conditions of equilibrium have been worked out, the appearance 

 or disappearance of certain minerals can be used as a sort of 

 geological thermometer, capable of indicating the limits within which 

 the temperature can have varied. 



The whole investigation is a splendid example of experimental 

 research devoted to a particular problem and directed by a well- 

 established theory ; the chemist in his laboratory has now succeeded 

 in tracing the changes that took place ages ago in the bed of a land- 

 locked sea as it laid down its contents and finally became a dry 

 basin, although he is not able to reproduce the original conditions or 

 to work for the long periods which Nature had at her disposal. 

 Without the logical consideration of the conditions necessary for 

 equilibrium, countless experiments might be made upon these salts, 

 and aa immense amount of speculation might have been devoted to 

 their possible reactions in the liquid state, about which we know so 

 little, instead of to their equilibrium when solidifying, about which 

 we know so much more. 



Some Petrographical Problems. 



The other geological problems which I have mentioned have also 

 been beyond the reach of actual experiment, for it is hopeless to 

 attain the immense pressures and high temperatures or the enormous 

 time that may have been required for the growth of natural minerals 

 in rocks and veins ; and so when difficulties are encountered there is 

 a tendency to ' explain ' them (if the word may be so misused) by 

 reference to the mysterious effect of conditions which cannot be 

 brought directly within the reach of experiment. 



I cannot help thinking that this has to some extent occurred in 

 the discussion of the petrographical problems which I propose to 

 consider next. There are two great liquid reservoirs from which 

 minerals have crystallised — the sea, with its dissolved salts, and the 

 subterranean baths of molten silicates, from which the igneous rocks 

 have been derived. It is true that in the sea two of the constituents, 

 water and sodium chloride, largely predominated over the others ; 

 but, after all, both sea and lava are liquids subject to the same 

 physical and chemical laws. 



