504 
experiment enlightened by the genius of Pasteur, Le Bel and 
Van ’t Hoff! That answer is that a twinned substance fresults, 
one indeed in most respects and chemically, but two in certain 
physical properties, characterised by presenting phenomena as 
of equal and opposite strains, a polarised pair of substances, in 
fact. What I mean by the double equipment with power is, of 
course, the pair of identically related and self-identical radicals, 
or the bivalency of one radical wholly and directly associated 
with the carbon radical. The case of the oxygen radical of 
aldehyde is that of the bivalent radical ; the other case is that 
of the two carboxyl radicals of hydroxytartronic acid, or that of 
the two methylene hydrogen radicals of alcohol which these carb- 
oxyls have replaced. The tetrahedral formula with its re- 
flected form admirably symbolises the case of enantiomorphously 
related pairs of substances. But no light whatever is thrown 
upon the nature of this pairing by the tetrahedron model ; its 
value depends upon the fact that as a symbol it so fully matches 
the constitution of the substances. 
Here I bring my summary of chemical theory and its formu- 
lation without hypothesis to a conclusion, hoping that, to some 
extent, 1 may have impressed you with the fact that the ex- 
position of even advanced chemistry, in its symbolic, equally 
as in its ordinary language and nomenclature, is independent of 
any hypothesis as to the mechanically and chemically differ- 
entiated structure of substances, and that chemistry can be 
studied and still further developed without reference to such a 
structure. I have asked for few or no reforms in the use of 
either terms or symbols, my point having been only to press fora 
consideration and discussion of the doctrines of chemistry and 
the great atomic theory itself as something concerned exclusively 
with experimental chemical facts. 
SECTION C. 
GEOLOGY. 
OPENING ADDRESS BY LIRUT.-GENERAL CHARLES ALEX- 
ANDER McManuon, F.RS., F.G.S., PRESIDENT OF THE 
SECTION. 
Rock Metamorphism. 
I wWIsH to offer some observations to-day on some aspects of 
rock metamorphism ; and as this is a complex subject, and the 
time at my disposal is brief, I purpose to deal with it in simple 
language, and to avoid as far as possible all petrological 
technicalities. 
A short description of a granite in the Satlej Valley of the 
Himalayas will, I think, introduce us by a short cut to the con- 
sideration of ‘* contact metamorphism,” an important branch of 
the subject under consideration. 
The granite [ allude to is an intruder in the normal gneissose- 
granite of the Himalayas, and cuts through it at right angles to 
its foliation. 
The intruder, which is some yards wide, did not rise through 
a simple crack or fissure, for its passage upwards was inter- 
rupted bya sheet of dark intrusive diorite, older than itself, 
which ran, roughly speaking, parallel to the foliation of the 
gneissose-granite. 
This sheet of diorite offered considerable resistance to the 
rising granite. 
The granite zigzagged backwards and forwards across the 
diorite and ran along its edges for fifty yards or more, converting 
it into a mica trap. 
It then tore itself away and continued its upward course. The 
granite I am describing was in a molten or fluid condition at 
the time of its eruption, as I hope to show in my subsequent 
remarks. 
I may pause here, however, to consider in passing what was 
the probable temperature reached by a granite such as that 
above described. 
The question is one of very great difficulty, as we know so 
little about the plutonic conditions of igneous rocks, and can 
only arrive at an answer to our question by indirect evidence. 
The melting point of quartz ranges from 1425° to 1450° C., 
but the fusion point of granite need not necessarily be as high 
as this, inasmuch as the presence of water at high temperature 
materially lowers the melting or solution point. 
The fusion point of the other constituents of granite may here 
be mentioned: that of orthoclase ranges from 1164° to 1168° ; 
microcline, 1169°; albite, 1172° ; augite and hornblende, 1188° 
to 1200° ; apatite, 1221°. Zircon, which is commonly found in 
NO. 1716, VOL. 66] 
NATURE 
[SEPTEMBER 18, 1902 
granites, and is one of the first minerals to separate out of the 
magma, is shown by Ralph Cusack to have probably a melting 
point of 1760°; whilst topaz, a not uncommon mineral in 
granite, is infusible up to the melting point of platinum, namely, 
1770 C. 
If we consider, therefore, the melting points of the mineral 
constituents of granite, we can hardly avoid the conclusion that 
for the magma to have attained perfect fluidity it must have 
reached a temperature of at least 1200° C. 
Vernadsky has shown that kyanite is transformed into silli- 
manite, a well-known product of contact-metamorphism at a 
temperature of 1320° to 1380°. 
If rocks in contact with granitic masses have been raised to 
this temperature, it follows that the granite itself must have 
been still more heated. Vernadsky’s observations have been 
relied on by Mr. George Barrow in his well-known paper ** On 
an Intrusion of Muscovite-biotite Gneiss”’ in the S.E. Highlands 
of Scotland to account for the presence of sillimanite in the 
inner zone of metamorphism between the kyanite schists and 
the granite, and he considered that the temperature attained by 
the ‘‘central masses of the Highland rocks” was probably 
higher than the figures indicated by Vernadsky. 
Bearing all considerations in mind, including the influence of 
water and alkali in reducing, and of pressure in raising, the 
melting point, I think we may safely infer that granites, such as 
the Himalayan granite alluded to above, must have been raised 
at plutonic depth to a temperature midway between red and 
white heat, that is to say, to at least 1200° C. 
To return to the granite of the Satlej Valley under considera- 
tion, I wish to draw attention to its condition just before crystal- 
lisation commenced. 
A study of the mineral beryl will, it seems to me, throw light 
on this point. 
Beryl is an important accessory mineral of the granite under 
description. It is clearly an original mineral, and it is material 
to note that it was the first mineral to crystallise out of the 
magma of the Satlej granite. This is shown by several circum- 
stances. 
In the first place the beryl preserved its perfect crystallo- 
graphic shape, showing that its molecules during the entire 
period of crystallisation possessed comparative freedom of 
motion, and were not interfered with or molested by other 
solid minerals. In the second place all the essential minerals 
of the granite when they subsequently crystallised out of the 
magma were deposited on the crystals of beryl. I have speci- 
mens of the granite showing crystals of beryl enclosed in felspar, 
in muscovite and in quartz. 
The beryl, therefore, having been the first mineral to crystal- 
lise, the examination of thin slices of it under the microscope 
ought to give us a clue to the condition of the magma at the 
time the beryl was formed. 
I have made such an examination, and I find that the beryl is 
crowded with liquid and gas cavities, the former containing 
movable bubbles and deposited crystals as well as water. 
The bubbles are of substantial size relative to the area of the 
cavities, showing that the water suffered considerable contraction 
after it was sealed up in the beryl. 
Scrope long ago suggested that the fluidity of lavas below the 
melting point was due chiefly to the water they contained, and 
attributed the liquidity of granite to the same cause. 
Scrope, however, in ascribing the mobility of an igneous rock 
to the presence of water, seems to have had regard principally 
or wholly to its mechanical action in furnishing an elastic 
medium in the interstices between the crystals or grains of the 
rock. He observes that a lava consists ‘‘of more or less 
granular or crystalline matter, containing minute quantities of 
either red-hot water, or steam in a state of extreme condensa- 
tion, and consequent tension, disseminated interstitially among 
the crystals or granules, so as to communicate a certain mobility 
to them, and an imperfect liquidity to the compound itself,” 
and he quotes Scheerer and Delesse, both of whom assert that 
water exists in mechanical combination with all crystalline 
rocks, ‘‘its minute molecules being intercalated between the 
crystals.” 
Nowadays one would attribute the liquidity of an igneous 
rock not so much to the mechanical action of the water present 
in it as to the combination of the water with the mineral 
contents of the lava, producing a state of solution. 
Sorby’s investigations supported Scrope’s observations, for he 
proved that the liquid contained in the inclusions in granite is 
