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THE NOMENCLATURE OF PETROLOGY



OTHER WORKS

BY

ARTHUR HOLMES

THE AGE OF THE EARTH.

Harper's Library of Living Thought.

HARPER BROS., LONDON AND NEW YORK.



THE GEOLOGICAL AND PHYSICAL CHARACTERS OF

CONCRETE AGGREGATES.
"Red Book" No. 256.

BRITISH FIRE PREVENTION COMMITTEE, LONDON.



PETROGRAPHIC METHODS AND CALCULATIONS.

Ready in the early Autumn.

T. MURBY AND CO., LONDON.



THE NOMENCLATURE
OF PETROLOGY

WITH REFERENCES TO SELECTED LITERATURE



BY

ARTHUR HOLMES

D.SC. (LOND.), A.R.C.S., D.I.C., F.G.S., F.R.G.S.

Demonstrator of Geology and Petrology in the Imperial College of Science and

Technology, London
Technical Geologist to the British Fire Prevention Committee



LONDON: THOS. MURBY & CO., i, FLEET LANE, E.G. 4
JSTEWYORK: D. VAN NOSTRAND CO., 8, WARREN STREET

1920




EARTH

SCIENCE*

LIBRARY



CONTENTS



PAGE

INTRODUCTION i

GLOSSARY OF TERMS 23

APPENDICES - 243

A, French Petrographic Terms - - 243

B. German Petrographic Terms

(compiled by Miss J.-H. ROBERTSON) 247

C. Greek Words and Prefixes - 257

D. Latin Words and Prefixes - 263

E. Classification Tables - - - 265



P255793



THE NOMENCLATURE OF
PETROLOGY

4

INTRODUCTION.

SOME years ago I began the compilation of a card-
catalogue of petrographic and associated terms, for
the use of students in the Geological Department of
the Imperial College of Science and Technology.
Each card gave a brief description of the meaning
(or meanings) of the term to which it was devoted,
together with references to those papers on the sub-
ject which were available in the departmental
library of the College, a library which, thanks to
the collections made by the late Professor J. W.
Judd and others, is unusually rich in author's
separates. As the catalogue grew, its general use-
fulness became apparent; and a series of sugges-
tions that it should be developed and published led
me finally to the conclusion that such a course
would not be unjustified.

There are many geological glossaries in which
petrological terms find a place, but they are for the
most part old, and to-day they are but little used, or



2 THE NOMENCLATURE OF PETROLOGY

even 'known. 1 Several petrological books contain
glossaries; notably Sir Jethro Teall's great work,
British Petrography (1888), and J. R. Kemp's
Handbook of Rocks. The latest edition of Kemp's
book was published in 1918, and contains a wealth
of information, particularly in relation to the older
terms and the newer American terms. The only
independent publication of the kind, however,
appears to be the Lexique Petrographique of
-Lcewinson-Lessing (Paris, 1901). This invalu-
able work is now nearly twenty years old, and so
luxuriant has been the growth of nomenclature dur-
ing the last two decades, that the Lexique no longer
serves as an adequate guide through the somewhat
tangled forest of names.

The complexity of petrological nomenclature at
the present day is demonstrated by the following
list, in which examples are given to illustrate the
varying characters and principles on which names
have been based from time to time.

Classical : basalt, basanite, obsidian, porphyry, syenite.
Popular : chert, cokeite, forellenstein, gabbro, gneiss,

granite, greisen, halleflinta, loess, marl, minette.
Structure: augen-gneiss, banket, cipolino, dermolith, folia-

1 Among those examined for the purpose of this book are the

following :
G. Roberts : An Etymological and Explanatory Dictionary

of the Terms and Language of Geology, 1839.
D. Page : Handbook of Geological Terms, 1859 & 1865.
W. Humble : Dictionary of Geology and Mineralogy, 3rd

Ed., 1860.

G. H. Kinahan : A Handy Book of Rock Names, 1873.
B. von Cotta (Trans, by P. H. Lawrence) : Rocks Classified

and Described, 1878.
T. H. Oldham : Geological Glossary, 1879.



THE NOMENCLATURE OF PETROLOGY 3

tion, knotenschiefer, lithophysae, oolite, perlite, pudding-
stone, rhyolite, schist, variolite.
Texture : anamesite, aphanite, granulite, hornstone, lithoi-

dite, pegmatite, rhomb-porphyry.
Roughness : grit, trachyte.
Colour : eclogite, graywacke, greenstone, leucocratic, leuco-

phyre, melanocratic, melaphyre, muscovadite, troctolite.
Lustre : euphotide, lamprophyre, pitchstone.
Fusibility : eurite, pyromeride, tachylyte.
Organic characters: coral-sand, crinoidal limestone, diato-

mite, globigerina-ooze, lignite, miliolite.
Mineral characters : aplite, diorite ;

albitite, amphibolite, anorthosite, argillite, augitite,

hornblendite, quartzite, peridotite ;

albite-enstatite rock, anorthite rock, muscovite-rutile

rock, quartz-barytes rock ;

glauconitic sandstone, glaucophane-schist, hornblende-
granite, mica-schist, nepheline-syenite, olivine-basalt,

quartz-monzonite, sillimanite-gneiss.
Chemical characters: alkali-rocks, anthracite, calc-alkali-

rocks, calc-flinta, calciphyre, picrite, soda-rhyolite.
Use : laterite, novaculite.
Mode of formation : crush-breccia, flow-breccia, mylonite,

stalactite.

Alteration : diabase, rapakivi.
Relative age : palasopicrite, proterobase, protogine.
Tribal names : gondite, ossypite.
Surnames : buchnerite, charnockite, dolomite, grahamite.

ulrichite.
Place-names : cornubianite, ivernite, norite ;

andesite, bostonite, canadite, jacupirangite, laurdalite,

monchiquite, nevadite, sussexite, tonalite, wyomingite.

Hunne diabase, Markle basalt, Ponza trachyte.
Compound rock-names: granodiorite, rhyodacite, syeno.

diorite, trachydolerite.
Greek prefixes: apo-rhyolite, epidiorite, hyalobasalt, kata-

gneiss, micropegmatite, orthogneiss, paragneiss, pseudo-

tachylyte.



4 THE NOMENCLATURE OF PETROLOGY

Greek suffixes: basanitoid, dacitoid, graneid, pegmatoid,

syenoid.
Mnemonics: felsic, femic, mafic, salic.

For many years the fashion has been established
of basing new rock-names on geographical names,
a method that burdens the memory with many ugly
and cacophonous terms, leads sometimes to re-
dundancy, and fails to suggest the distinctive
characters of the rock-types so described. It is
difficult, however, to see how these objections can
be altogether avoided. A different application of
the method has sometimes been made, a new type
being described partly in terms of a well-known
rock-name, and partly in terms of the locality where
the type-rock occurs. Thus we have Ponza
trachyte, Hunne diabase, and Markle basalt. More
purely descriptive names, formed by adding
mineral-prefixes to existing rock-names, such as
biotite-hornblende-granite, are self-explanatory;
and the same advantage is shared by compound
terms like granodiorite, trachyandesite, and melano.
cratic olivine-trachydolerite. There is much to be
said in favour of combinations of these kinds, as
they reduce the number of fundamental names to be
remembered, and are of wider application than
specific names. Many protests have been made
against the use of long compound-names, but,
unless they become ridiculously cumbrous, they are
thoroughly justified in the interests of clearness, as
they are, for example, in organic chemistry.

A geographical appellation already established,
such as lugarite or marloesite, should not be



THE NOMENCLATURE OF PETROLOGY 5

adopted for a rock from a fresh locality, unless the
identity of type so implied is sufficiently close to
avoid all chance of misconception. On the other
hand, a new name should not be resorted to until
every other possibility has been tested and found
inadequate. There is undoubtedly an attraction in
the creation of new names, and in too many cases
that attraction has not been dispelled by the verbal
discords eventually produced. On several occa-
sions in my own experience I have been inclined to
coin specific names. A riebeckite-asgirine granite
from Angola 1 was a temptation for a time, but fortu-
nately it was resisted. Otherwise our nomencla-
ture would have been burdened with two new and
unnecessary synonyms, for simultaneously Lacroix
described a similar rock from Madagascar 2 under
the name fasibitikite. In a case like this I consider
that three words are better than one. Brevity of
expression is by no means an unmixed blessing,
and the one word may require a whole paragraph of
explanation.

It would, of course, be desirable if definitions of
rock-names could be framed by an International
Committee endowed with authority to fix meanings
finally, and to decide on the validity of new terms
at suitable intervals. Unfortunately such a counsel
of perfection is not likely to be sought for many
years, and even were a powerful committee to be
formed, its authority would sooner or later be
sapped by disagreement. One such attempt to

1 A. Holmes : Geol. Mag., 1915, p. 267.

2 C.R., clxi, 1915, p. 253.



6 THE NOMENCLATURE OF PETROLOGY

standardise nomenclature revealed so wide and
stubborn a divergency of opinion as to its prac-
ticability, and the individual rights of authors to
use terms as they choose, that no final decisions
were arrived at, and only a few general suggestions
and the revised Lexique of Lcewinson-Lessing
emerged from the conferences. 1 The authors of
the Quantitative Classification of Igneous Rocks
have summed up the position by a quotation so
happy that no apology is necessary for repeating it
here.*

" There's glory for you," said Hnmpty Dumpty.

" I don't know what you mean by glory," Alice said.

Humpty Dumpty smiled contemptuously. " Of course you
don't till I tell you. I meant there's a nice knock-down
argument for you !"

" But glory doesn't mean ' a nice knock-down argument,' "
Alice objected.

" When / use a word," Humpty Dumpty said in rather a
scornful tone, " it means just what I choose it to mean
neither more nor less."

" The question is," said Alice, " whether you can make a
word mean so many different things."

" The question is," said Humpty Dumpty, " which is to be
master that's all."

Alice Through the Looking Glass (Lewis Carroll).

Two main difficulties stand in the way of uni-
versal agreement. One of these is the natural
tendency of words in active use to grow, and

1 Com-ptes Rendus, viii, Congres Geologique International,
Paris (1900), 1901.

2 W. Cross, J. P. Iddings, L. V. Pirsson, & H. S. Washing-

ton : Journ. Geol., xx, 1912, p. 559.



THE NOMENCLATURE OF PETROLOGY 7

gradually to assume a wider and therefore less pre-
cise meaning than that for which they were
originally intended; while in time they may come
to have a totally new, and at first sight quite un-
related, meaning. The standard petrological
example of a word illustrating this process is
porphyry ; and quite recently the term dedolomitisa-
tion began to extend its scope, though in this case
the tendency was thwarted by a timely protest. 1
Even in 1811 we find Pinkerton complaining in his
Petralogy that the term freestone, instead of being
restricted to "the noblest of the common lime-
stones," had been inaccurately applied to sand-
stones! He overcame this "abuse of language"
by proposing konite for the limestones so beloved
of the mediaeval freemasons. And introducing
another rock which he describes with great
enthusiasm, he writes, "This is the celebrated
pudding-stone of England, so much in request in
foreign countries; but this name commonly excit-
ing a smile among the illiterate, and the applica-
tion being since enlarged to a great number of
glutenites, 2 of a different nature and origin, form-
ing entire chains of mountains (while this is con-
fined to a very small district in England, and is
found nowhere else in the world), it has been
thought proper to distinguish it by the name of
Kollanite; derived from the Greek, denoting its
appearance of being cemented together."



1 Geol. Mag., 1919, p. 458.

2 A term, mow obsolete, for breccias, conglomerates, and

sandstones.



8 THE NOMENCLATURE OF PETROLOGY

The late A. D. Darbishire in his posthumous and
unfinished Introduction to a Biology (1917) dis-
cussed the wanderings of words in relation to a
subject where the danger is even greater than in
ours.

" If there is a possibility that words give a
semblance of progress in interpretation, where in
reality there is none, it is desirable that some atten-
tion should be paid to the relation between word
and thought. . . .

" A word and its meaning, especially in the case
of ideas, are united together by a slender, elastic
bond which is now contracted, now stretched to its
uttermost. ... So we see the word and its mean-
ing dancing to each other in an airy medium, like a
pair of gnats in the lee of a gorse-bush. This,
alas ! is the simplest case. The more complicated
and much more common cases are those in which
one word has more than one meaning, or where one
meaning has more than one word to express it;
these are the cases which, in verbal life, are pro-
ductive of trouble " (pp. 19-21).

Such difficulties are sometimes accentuated by
human perversity : witness Rosenbusch's treat-
ment of Vogelsang's term granophyre, and
Brogger's appropriation of foyaite and ditroite.
Consider also the introduction of the terms aa and
aphrolith to denote the qualities already simply ex-
pressed by block-lava-, and of midalkalite and
syenoid to take the place of nepheline-syenite.

The second difficulty standing in the way of
agreement is intimately related to the first.



THE NOMENCLATURE OF PETROLOGY 9

Although petrology is now developing rapidly, no
generally accepted classifications of the major
groups of rocks have yet been devised with suffi-
cient detail to guide the choice of terminology and
to restrain its tendency to spread outwards and be-
come confused. Existing classifications involve a
and chemical composition, proportions of minerals,
and chemical composition, proportions of minerals,
structure, texture, mode of occurrence, degree of
alteration, etc. and any given name is therefore
liable to wander in various directions according as
insistence is placed on one or other of its possible
connotations. A successful classification will need
to be sufficiently elastic to avoid the despotism of
merely arbitrary and unsignificant division, and
yet sufficiently rigid to standardise the meanings of
its collateral terminology.

At the present time the field of petrology still
contains many uncultivated corners, and until the
whole has become familiar ground, existing
systems of classification and nomenclature must be
regarded as on probation. It is my impression that
stability will be approached, not primarily as a
result of any committee, international or sectional,
but by the co-ordinating work of a single petrolo-
gist of genius whose authority, the outcome of his
own success and influence, will be far superior to
the merely temporising and democratic authority
of a committee.

Meanwhile it seems desirable to take stock, and
to place on record the existing nomenclature in
accordance with its current usage. It is hoped that



io THE NOMENCLATURE OF PETROLOGY

this book will meet the need by creating a standard
of reference which may to some extent limit future
vacillations, and prevent unnecessary clashing
between new terms. The main object of the book
is, however, to be practically useful by serving as a
guide to student, teacher, research-worker, and
professional geologist, and indeed to all who need
to follow petrological literature or to contribute to
its pages.

The work involved in revising and amplifying
the original card-catalogue, undertaken about a
year ago, proved to be more arduous than
was at first anticipated. It has, however, never
descended to drudgery or mere compilation. On
the contrary it has been in the nature of a literary
exploration, leading one to examine a century's
evolution of petrological thought and method, and
to share the delights of many a curious traveller
through little-known corners of lands the world
over. Unsatisfactory though certain parts of the
nomenclature may be as an instrument of thought
and exposition, it is, as a w^hole, unusually rich in
pleasant associations, geographical, historical, and
even psychological. Perhaps this romantic aspect
of a subject bristling with technicalities is a dan-
gerous one, for it tends to support the natural con-
servatism of even scientific men, and so, perhaps,
to retard the development of that ideal system of
nomenclature which we all hope for but cannot as
yet create.

Certain attempts to systematise nomenclature
have already been made, particularly in the field of



THE NOMENCLATURE OF PETROLOGY n

igneous rocks. It is desirable to draw attention to
some of these, because, with a few exceptions, the
terms they comprise have been excluded from the
glossary that follows.

Jevons suggested a wholesale use of prefixes con-
sisting of contracted, or rather mutilated, forms of
structural, textural, and mineral names, together
with a few chemical and qualifying syllables. 1 He
thus arrived at such combinations as ophit-oli-
dolerite, diopsi-mipegmo-rhyolite, rhomfels-pyr-
alisyenite ( = Laurvikite), and eud&gi-midalkalite
( = Lujaurite). Such proposals are obviously fore-
doomed to failure. In the words of Professor
Bonney, "Time is not so valuable, or paper and
printing so expensive, that we should talk or write
4 gibberish ' to save a few letters."

A more reasonable method has recently been
proppsed by Professor Shand, 2 based in its applica-
tion on the classification of igneous rocks according
to his principle of saturation. 3 He suggests

(1) That the names of oversaturated and saturated
rocks should end in the customary suffix -He ; e.g.,
granite, syenite, etc.;

(2) That the names of unsaturated rocks should
carry

(a) the prefix sub-, or the suffix -ole, to indicate
that the dyad or triad metals are unsaturated ;
e.g., subgabbro, for olivine-gabbro ;

(b) the suffix -oid, to indicate that the monad



1 H. S. Jevons : GeoL Mag., 1901, p. 304.

2 S. J. Shand : Geol. Mag., 1917, p. 466.

3 GeoL Mag., 1913, p. 508; 1914, p. 485; 1917, p. 115.



12 THE NOMENCLATURE OF PETROLOGY

metals are unsaturated; e.g., syenoid, for
nepheline-syenite ; and

(c) a combination of the prefix sub- and the
suffix -old, to indicate that both monad and
dyad metals are unsaturated, e.g., subthera-
loid or subgabbroid, for olivine-theralite.
The principle adopted is excellent, but the choice
of suffix, -aid, is unfortunate and cannot be
accepted, for it has already been seriously over-
worked in other directions. It has been used in
adjectival terms like granitoid and trachytoid, tc-
express texture or composition ; and in substantive
form in the term pegmatoid, to denote very coarse-
grained facies of igneous rocks differing from peg-
matite proper by the absence of graphic-texture;
and in terms of which dacitoid is a typical example,
to connote similarity (to dacite) of chemical com-
position combined with dissimilarity of mineral
composition.

The authors of the Quantitative Classification
have introduced a very comprehensive nomencla-
ture, the greater part of which is built up with the
aid of a variety of suffixes and mnemonic contrac-
tions. Terms like felsic and mafic are extremely
useful, and even though they have been regarded
as technical slang, they have justified their inven-
tion by having been widely adopted. On the other
hand, more ambitious and less useful terms such as
alferfemphyric are ugly and have not met with a
similar measure of favour. Cross, Iddings,
Pirsson and Washington 1 have themselves made

1 These authors are referred to in the glossary by CJ.P.W,



THE NOMENCLATURE OF PETROLOGY 13

the recognition of much of their nomenclature a
necessity, for they have forcibly and persistently
made use of it in a long series of publications which
other petrologists cannot afford to ignore. Most of
the new terminology, however, is intimately related
to, and only used in connection with, the Classifica-
tion itself, and with the latter it must therefore stand
or fall. Unfortunately, the principles on which the
Classification is based leave the main problems of
petrology untouched, fail to open out new fields of
research, and therefore do not constitute a creative
contribution to the subject they were intended to
illuminate. From this point of view the apparently
wide influence exerted by the Classification in
recent years has been largely factitious. Neverthe-
less, it is only fair to add that the authors of the
Classification have rendered very real services to
petrology by promoting greater accuracy of
description and analysis, and by introducing the
conception of the norm, which provides an admir-
able method of recalculating, comparing, and inter-
preting rock-analyses.

Another systematic terminology to which refer-
ence must be made has he_en proposed and exten-
sively used by Grabau in his Principles of Strati-
graphy (1913). The terms are summarised on
pp. 296-7 of that work, and constitute an attempt,
laudable in principle, to provide a comprehensive
nomenclature for sedimentary and associated rocks.
By means of a number of prefixes representing
chemical or mineral composition and agency of
formation, compound terms are built up at will by



i 4 THE NOMENCLATURE OF PETROLOGY

combining them with a series of grade designa-
tions : rudyte, corresponding to gravel, shingle,
pebbles, etc.; ar&nyte, corresponding to sand; and
lutyte, corresponding to mud or rock-flour. Thus
anemoarenyte in ordinary terms would be described
as asolian sand; hydrosilicirudyte as quartz-con-
glomerate; and pyrolutyte as volcanic ash or
dust. The extent of departure from current
nomenclature is unnecessarily wide, and it seems
doubtful whether such innovations will ever be
recognised by adoption. Grabau's use of the terms
exogenetic and endogenetic is particularly unfortu-
nate and tends to confusion of thought. 1 He
describes rocks as "exogenetic" when they have
been formed by agents acting from without, that is,
acting externally with respect to, and indepen-
dently of, the finished rock, as in the case of loose
detrital sediments. Other rocks, formed by agents
acting from within, he describes as "endogenetic,"
this category including igneous rocks, saline
deposits, and organic accumulations. The two
contrasting terms, although they are applied to
rocks, are thus made to be synonymous with
allo genie and authigenic respectively, and as the
latter terms lead to a far clearer realisation of the
primary division proposed by Grabau there seems
to be no reason for rejecting them. The sentence
" A calcareous sandstone contains allogenic grains
held together by an authigenic cement," gives an
accurate statement of fact, whereas the classifica-

1 Amer, Geol., xxxiii, 1904, p. 228,



THE NOMENCLATURE OF PETROLOGY 15

tion of a calcareous sandstone as " exogenetic " in
Grabau's sense expresses only part of the truth.

The obvious and most serviceable meanings of
exogenetic and endogenetic are those proposed by
Mr. T. Crook, 1 and they should be adopted and
used as defined by him. Exogenetic applies
to processes originating and operating at or near
the earth's surface, and to the rocks and ore-
deposits formed by such processes. Endogenetic
applies to processes originating internally and
operating deep-seatedly in the earth's crust, or from
within outwards, and to the rocks and ore-deposits
formed by such processes.

If it be objected that Grabau has priority as re-
gards date of publication, the reasons for rejecting
his usage would be based on the following points

(a) The French equivalents of the terms,
exogene and endogene, have long been used
to express the division of rocks into ' * erup-
tive " and " sedimentary " groups.

(b) Reference to Murray's Oxford Dictionary
will show that exogenetic had previously
been recognised in the sense followed by
Crook.

(c) Grabau has used the terms for a concep-
tion which they fail adequately to express,
and for which wholly adequate terms were
already available.

Only one of Grabau's terms, rudaceous, has been
included in the glossary, because with arenaceous

1 " The genetic classification of rocks and ore deposits,''
Min. Mag. t xvii, 1914, p. 72.



16 THE NOMENCLATURE OF PETROLOGY

and argillaceous, it completes a Latin trilogy for
the three main groups of detrital sediments. As
far as I know there has not hitherto been a term of
Latin form for coarsely graded detritus of the kind
which is described in the corresponding Greek
trilogy as Psephitic.

Several of the Cross-Iddings-Pirsson-Washing-
ton terms have been incorporated, including a


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