TRANSACTIONS OF SECTION C. 379 
In reconstructing the history of these deposits account must also be taken of 
the varying vapour pressures of the solutions which are saturated with the 
different compounds, as this really determines 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 fortundtely this is 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 léweite, 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 48°; in fact, 
after the conditions of equilibrium have been worked out, the appearance or dis- 
appearance 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 experi- 
ments might be made upon these salts, and an 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 lave 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 diffi- 
culties 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 con- 
stituents, water and sodium chloride, largely predominated over the others; but, 
a all, both sea and lava are liquids subject to the same physical and chemical 
aws. 
An admirable summary of the evolution of petrographical ideas was given in 
the Presidential Address to the Geological Society of London in 1901 by Dr. 
Teall, who dealt both with the consolidation of rocks from molten magmas and 
their differentiation into species. It is not, therefore, necessary for me to con- 
sider anything but recent work which has been done during the last four years, 
and the earlier controversies may be left out of account. 
Among the many problems relating to the mineral and chemical constitution 
of rocks which have yet to be solved, two, and those perhaps the most important, 
should lend themselves most readily to experimental treatment. The first is the 
problem of rock differentiation : why does a magma, even one which has pre- 
sumably crystallised in situ, separate itself into zones, or layers, or streaks of 
different constitution? And the second is the problem of mineral differentiation: 
why does a granite magma, for example, crystallise as a mixture of the particular 
