72 SECTIONAL ADDRESSES. 
(b) The only igneous rocks that consolidate from high temperatures 
at atmospheric pressures with free escape of contained gases are the 
voleanic lavas. The plutonic and intrusive rocks consolidate under high 
pressures and with retention of their gases. Hall began his experiments 
with dry melts in open furnaces; but he realised that under these 
conditions it was not possible to crystallise a marble. The historical 
development of research has followed similar lines, and investigations 
under pressure are now becoming more prominent. In the special field 
in which Hall worked we owe very important results to Professors 
Adams and Nicolson, of Montreal. They experimented on the effects 
of very high pressure (obtained by a hydraulic press) on chalk, lime- 
stone, and marble. Columns of limestone were embedded in alum or 
fusible metal enclosed in a steel tube, and submitted to enormous pres- 
sures. In some of the experiments the apparatus was heated to 300° or 
400°C., and that the investigation was on ‘ the effects of heat modified 
by compression’ as stated in the title of Hall’s original paper of 1805. 
They succeeded in obtaining plastic deformation in the solid rock, with 
development of schistosity and cataclastic structures but without exten- 
sive recrystallisation. These experiments illustrate very perfectly the 
formation of such rocks as cale-schists, mylonites, flaser-gabbros, and 
augen-gneisses. 
It is generally agreed by physicists that increase of pressure makes 
little difference on the melting-points of solids, and that a slight rise 
of temperature may have a much greater effect on the stability of a 
mineral system than a considerable rise of pressure. But this is to 
some extent altered where volatile substances are concerned, for then 
pressure modifies the concentration, offen to a high degree. In many 
rocks, and especially the acid plutonic rocks and mineral veins, the 
importance of volatile mineralisers is abundantly clear. In the crystal- 
line schists, on the other hand, we see the effects of pressure, not only 
in the structures of the rock masses, but also in the special minerals 
which characterise this group. The importance, accordingly, of pressure 
and volatile components cannot be ignored, and experimental petro- 
logists are now directing their attention especially to a study of their 
influence. The ground has been cleared by a masterly series of 
mathematical researches, principally by Schreinemakers and Smits, 
and experimental work along these lines is rapidly advancing. 
(c) The third agency which nature employs in the making of rocks 
but is apt to be neglected in the laboratory is time. It is not always 
easy to estimate its importance. A laboratory experiment under excep- 
fional circumstances has been carried on over several months; most 
of them are finished in a few hours, but nature works with un- 
limited time. The action of solvents when they occur in very small 
quantities is favoured in this way and unstable phases tend to dis- 
appear. In the deposition of mineral veins this may be a factor of 
paramount importance, and it also cannot be ignored in all studies of 
metamorphism and metasomatism. 
We learn from Hall’s papers that he was continually experimenting, 
and he delighted to devise means to put geological theories to ptactical 
tests. 
