SEPTEMBER 18, 1902] 
NATURE 
595 
water, and showed that it was caught up during the formation 
of the crystals, ‘‘ and was not introduced subsequent to the con- 
solidation of the rock.” 
The water now contained in cavities in the beryl was-probably 
held in solution by the constituents of that mineral at the time 
of its formation, and as it cooled down the water separated from 
the substance of the beryl and formed the cavities in which we 
now find it imprisoned. 
If this be so, it follows that when the beryl crystallised out of 
the magma, the latter was in a fluid condition, and held a con- 
siderable amount of heated water in solution. The temperature 
of the magma must have been above that of red heat, and the 
potential energy of the water held ina fluid state by pressure 
must have been great. When therefore in the course of the 
earth movements which accompany or in some cases are caused 
by the intrusion of eruptive igneous masses, pressure was tem- 
porarily relieved by the rupture and faulting of rocks, the super- 
heated water contained in the magma would be ready to flash 
into steam with almost explosive violence. 
It must also be borne in mind that water under great pressure, | 
at or above a red heat, has a powerfully solvent action on most | 
minerals, even on so refractory a mineral as quartz. When 
therefore granite in the molten and fluid condition of the Satlej | 
granite was erupted along a line of faulting, fissure, or weakness, 
the superheated water or steam, bearing with it much mineral 
matter in solution, must have acted with great chemical energy 
on the rocks into which it was intruded. 
I have spoken of water carrying mineral matter in solution, 
and of a magma carrying water in solution. 
ditions may rapidly succeed each other under varying conditions 
of temperature and pressure. To use the words of Van Hise, 
‘under sufficient pressure and at a high temperature there are 
all gradations between heated waters containing mineral ma- 
terial in solution and a magma containing water in solution.” 
The condition of the beryl crystals, crowded as they are with 
liquid cavities, shows how high a proportion of superheated 
water was contained in the fluid granite magma at the time of 
their formation. 
Sorby estimated that the fluid cavities in the quartz of granites 
sometimes amount to more than ten thousand millions to the 
cubic inch. As quartz, however, is usually the last mineral of 
a granile to consolidate, it may be thought that the water con- 
tained in it isa residuum left by the felspar and muscovite on 
their separation from the magma ; but the case of the beryl 
above quoted shows clearly that the amount of water diffused 
through the magma before the mica, felspar and quartz began 
to consolidate must have been very considerable. The amount 
of water held in solution by a granite, during the time of its 
aqueo-igneous fusion, cannot be estimated by the amount of 
water given in the analysis of consolidated and dried hand- 
specimens of that rock. A considerable proportion of this 
liquid must necessarily have been lost during the gradual cooling 
of the rock, and in the course of its intrusion into neighbouring 
sedimentary strata as sheets, dykes and veins. Sorby, as the 
result of other lines of investigation, came to the conclusion that 
the amount of water present in granite, though limited, is con- 
siderable. 
We must now turn for a few minutes to consider the important 
question of the porosity of minerals, and their permeability by 
heated water and gas at high pressure. 
The fact that solid substances are built up of molecules having 
interstitial spaces between them hardly needs demonstration 
nowadays. 
But have we all quite realised that the molecules of rock- 
forming minerals and crystals are not inert particles of matter, 
but that they vibrate or revolve or are endowed with other 
orderly movement that may be likened to the motion of the 
planets round the sun ? 
Far, far away in space the solar system would, to an eye 
formed like our own, in all probability present a nebulous ap- 
pearance, because the eye would not be able to see the individual 
members of our system. 
So, too, the molecules of which crystals are built up may have 
their appropriate motions, but we cannot see them with the eyes 
of sense because the molecules are beyond the highest powers of 
the microscope. 
We can, however, I think, perceive them with the eye of the 
scientific imagination ; and the hypothesis that the molecules of 
minerals are separated from each other by intermolecular spaces, 
NO. 1716, VOL. 66] 
These two con- | 
and have their modes of motion, seems essential to the compre- 
hension of rock metamorphism. 
The important experiments of Sir W. Roberts-Austen on 
the diffusion of gold in pure lead throw considerable light on 
this subject. 
Disks of solid gold were held against the bases of cylinders of 
lead by clamps, and were kept in an upright position at the 
ordinary temperature for four years. At the end of this time it 
was found that the gold had diffused upwards in the solid lead, 
for a distance of 7°65 mm., in sufficient quantity to be detected 
by the ordinary methods adopted by assayers. Traces of gold 
were found still higher. 
When a column of molten lead, 16cm. high, was placed above 
solid gold and kept ata mean temperature of 492° C, that is to 
say, at 166° above the melting point of lead, but 569°7° below 
that of gold, the gold diffused in considerable amount, to the 
top of the lead column, in a single day. 
Sir W. Roberts-Austen’s experiments, above alluded to, 
demonstrate that even such metals as gold, whose melting point 
is as high as 1061°7° C., exhibit a measurable amount of kinetic 
energy at the ordinary temperature and pressure. Great results 
may no doubt be brought about at ordinary temperatures and 
pressures, when time, as in the laboratory of nature, is practically 
unlimited ; nevertheless the importance of high temperature and 
high pressure, in operations connected with metamorphism, can 
hardly be overrated. 
Not only does a rise in temperature increase the energy of 
the chemical actions and reactions which produce the mineral- 
ogical changes embraced by the term metamorphism, but it 
increases the porosity of minerals and facilitates the passage of 
liquids and gases through their pores. 
The cohesion of molecules is lessened, the amplitudes of their 
vibrations, rotatory or other movements, are increased, and a 
passage is opened for the advance of chemical materials into the 
heart of the crystal. 
Increase of temperature thus not only throws open the doors 
of the mineral fortress attacked, but gives enhanced energy to 
the invaders. The fact that the mineral components of a rock 
are, under conditions of heat and pressure practically porous 
to heated water, laden with chemical reagents in solution, is 
frequently brought home to the mind of the petrologist in a very 
tangible way. We sometimes observe, for instance; that 
metamorphic changes begin at the heart of a crystal, and leave 
the peripheral portions of it fresh and unaltered. 
In such cases the chemical agents of change have evidently 
passed freely through the outer parts of the crystal, and have by 
preference selected its internal parts for attack. 
In order to explain clearly how this remarkable result takes 
place, in the cases referred to, it will be necessary to diverge for 
afew minutes to consider another branch of our subject. It is 
difficult, if not impossible, to lay down any hard-and-fast rule 
of universal application, because the conditions under which 
igneous rocks crystallise vary with temperature, pressure, the 
relative proportion of constituents and other local causes, and 
these variations in the conditions may materially affect the re- 
sults; but I think the rule that minerals crystallise out of a 
molten magma inthe order of their basicity is of very frequent 
if not of absolutely general application. This rule also governs 
the growth of individual crystals, especially those that exhibit 
what is known as zonal structure. Take, for instance, the felspars 
of an igneous rock, A gradual passage may frequently be traced 
by the petrologist from one species of felspar at the heart of a 
crystal to another distinct species at its periphery. Sometimes a 
crystal is made up of more than two species, which shade more or 
less gradually into each other. In accordance with the rule laid 
down above, the more basic species formed first ; then, as the 
percentage of the bases left in the magma gradually decreased, 
owing to the first formed crystals having taken a lion’s share of 
the available bases, the felspars that formed later became 
gradually more and more acid in composition. Thus a large 
felspar of slow and gradual growth may be composed of several 
zones, each zone being successively less basic and more acid 
than that upon which it crystallised, each successive zone thus 
possessing slightly different physical properties from the one that 
formed beforeit. These statements are capable of proof. When 
sections of felspar, such as occur in thin slices of igneous rock, 
are examined under the microscope in polarised light, petrolo- 
gists can distinguish one species from the other—when the 
direction in which the sections were cut is approximately known 
