Modern petrography. An account of the application of the microscope to the study of geology online

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PUBLISHER'S PREFACE.



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invested.

Of this series we are now ready to announce the four following :

Modern Petrography.

By GEORGE HUNTINGTON WILLIAMS, of the Johns Hopkins Uni-
versity. Price by mail, 25 cents.

The Study of Latin in the Preparatory Course.

By EDWARD P. MORRIS, M.A., Professor of Latin, Williams Col-
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MODERN PETROGRAPHY



AN ACCOUNT OF



THE APPLICATION OF THE MICROSCOPE
TO THE STUDY OF GEOLOGY



BY

GEORGE HUNTINGTON WILLIAMS

\ i

ASSOCIATE PROFESSOR IN THE JOHNS HOPKINS UNIVERSITY




BOSTON

D. C. HEATH & CO., PUBLISHERS
1886



COPYRIGHT, JULY 10, 1886
BY D. C. HEATH & Co.



ELECTROTYPED
BY C. J. PETERS AND SON, BOSTON.




MODERN PETROGRAPHY.

AN ACCOUNT OF THE APPLICATION OF THE MICROSCOPE
TO THE STUDY OF GEOLOGY.



IT cannot be denied that the terms Petrography and Lith-
ology, which only within very recent years have come to
occupy a really important place in American geological liter-
ature, still convey but a vague meaning to most teachers of
natural science. Many men who have devoted themselves
altogether to the study of geology know little of the origin,
aims, or capabilities of the youngest branch of their profes-
sion. That the scientific study of the crystalline rocks has,
during the past twenty-five years, rapidly developed in Ger-
many is a fact of which any one may easily convince himself.
The perfection of its methods already vies with that in many
other older departments of investigation ; while the impor-
tance of its results have long since secured for it a well
recognized place among the descriptive sciences. Nor has
its value as an educational discipline been overlooked.
Nearly all of the German universities have to-day, if not their
special professor of petrography, as may be found at Heid-
elberg, Munich, Leipzig, Berlin, and Vienna, at least their
regular courses of lectures on this subject, and their labora-
tories amply equipped for its pursuit.

It is only quite recently, however, that the importance of
what the Germans have accomplished in this direction has



4 MODERN PETROGRAPHY.

commenced to be appreciated in this country. When the
United States Geological Survey of the 4oth Parallel, undei
Mr. Clarence King, found it necessary to have a systematic
study made of its collections of crystalline rocks, there was
no American prepared to undertake such a task, and the
work was intrusted to Professor Zirkel of the University of
Leipzig. The appearance of the results of his labors in an
admirably illustrated quarto volume, entitled Microscopical
Petrography of the Rocks of the Fortieth Parallel, published as
Vol. VI. of the reports of the survey, in 1876, first opened
the eyes of most geologists in America to the new and prom-
ising field of research. Since that time the interest mani-
fested in this line of study in America has been steadily
on the increase. The geological surveys of this country
are already realizing the great value of accurate petro-
graphical studies; and if, indeed, we but compare the
quantity and quality of unexplored material in America with
that in Europe, we must conclude that it is here that the
study of rocks is destined to reach its highest development.
Much that is excellent has already been accomplished on this
side of the Atlantic; but the workers are few, and hereto-
fore there has been observable too little of the rigid scientific
accuracy which comes only after years of patient labor. We
are, however, heirs of the past, and it is only fair that we
should profit by all the accumulated experience of our prede-
cessors. What is above all things necessary to those entering
upon a line of research so difficult and new is a careful
training in what has already been discovered, that labor may
not be spent in vainly working out results which have already
been attained by others. To judge from the American stu-
dents * who, during the past six years, have done more or less

* During the writer's residence in Heidelberg, 1880-1883, nearly one-
half of all the students in the petrographical laboratory were Americans*
and the proportion now is even greater.



MODERN PETROGRAPHY. 5

work in the petrographical laboratories of Germany, the
importance of such a training has not been overlooked ; and
the constantly increasing value of American petrographical
work abundantly justifies the expenditure of time necessary
to secure it.

Until recently, it has not been possible for a student to
secure a satisfactory preparation in microscopical petrogra-
phy without going abroad for it. The English language does
not yet contain a satisfactory text-book* on this subject,
although, as we shall see, the first idea of applying the
microscope to the study of rocks originated in England.

Regular instruction in petrography has for some years
past been given at Harvard and Columbia Colleges ; more
recently, the attempt has been made at the Johns Hopkins
University to organize a petrographical laboratory, where, by
lectures and practical work, graduate students of geology
may secure a thorough acquaintance with all the methods
and results of foreign investigators. The encouragement with
which this experiment has already met seems to indicate that
it fills a need. Nor are signs wanting that other American
universities mean to follow this lead by introducing instruc-
tion in petrography among their courses.

In view, therefore, of the steadily increasing interest in
this new branch of geological research, it has been thought
that a brief account of the origin and history of microscopi-
cal petrography, as well as of some other methods of rock-
investigation to which its cultivation has given rise, might

* Lawrence's translation of Von Cotta's work, Rocks Classified and
Described, London, 1866, contains no allusion to the microscope; while
the small text-book by Rutley, The Study of Rocks, 1879, is to inaccurate
and too short to be of much use. Nor can more be said in favor of the
recent translation of Dr. Hussak's book, The. Determination of Rock-
forming Minerals. The German edition of this work is not satisfactory
in its arrangement or reliable in its statements, and the translation, in-
stead of being an improvement, is rather worse than the original.



6 MODERN PETROGRAPHY.

not prove unwelcome to teachers in many departments of
natural science.*

The reason why petrography has so recently sprung into
prominence is not because its importance was not early
recognized, but rather on account of its great practical diffi-
culties, which have only within the past two decades been
successfully overcome. The fierce contests between Nep.
tunists and Vulcanists, from which the very science of geol-
ogy sprung, themselves hinged largely on different hypotheses
regarding the nature and origin of crystalline rocks. The
followers of each school strained every nerve to fortify their
position, and the new sciences of chemistry and mineralogy
were made to contribute their utmost to both sides. Much
was speedily learned about the composition and mineral con-
stituents of the coarse-grained rocks, but any satisfactory
information regarding those which were fine-grained, and
apparently homogeneous, eluded the search of even the most
thorough and patient investigators. It was, however, about
exactly this class of rocks that the discussion had been most
bitter, and we can but regard with admiration the time and
study which the ablest geologists devoted to them. Still, the
results attained were very small. In 1815, Cordier finally

* Those desiring more detailed information on this subject will do
well to consult:

H. Fischer \ Chronologischer Ueberblick iiber die allmahliche Einfuhr-
ung der Mikroskopie in das Studium der Mineralogie, Petrographie, und
Palaeontologie, 1868.

F. Fouque, La petrologie en Allemagne. Revue scientifique, 1875, No. 34.

F. Fouque, Les applications modernes du microscope a la geologic.
Revue des deux mondes, July 15, 1879.

F. Zirkel, Die Einfiihrung des Mikroskops in das mineralogisch-
geologische Studium. Leipzig: 1881.

A. Stelzner, Die Entwickelung der petrographischen Untersuchungs-
methodenin den letzten fiinfzig Jahren. (Isis Festschrift.) Dresden: 1885.

/./. H. Teal, The Scope and Method of Petrography. Nature, March
12, 1885.



MODERN PETROGRAPHY. 7

proved that basalt was an aggregate of several minerals,
which he succeeded in partially isolating by most laboriously
washing them with water. This discovery was greeted with
delight, but the method was at the same time recognized as
incapable of general application. Aside from the immense
amount of time which it required, it was, at best, exceedingly
imperfect, on account of the slight differences in the specific
gravity of the component minerals. As a matter of fact, it
was never used again.

The rapid development of analytical chemistry during the
first half of this century threw much light on the relations of
different crystalline rocks, but even this could not furnish
what geology most needed a means of determining, with
certainty, their mineralogical components and structure. The
interpretation of analyses was necessarily vague, since they
could be calculated to satisfy many different aggregates of sil-
icate minerals.

In the light of our present knowledge, we wonder, not at
how little, but at how much, the patience and acumen of men
like Von Buch, Brongniart, Cordier, and Naumann were able
to discover in regard to fine-grained rocks with the exceed-
ingly primitive means at their disposal. Their wide experi-
ence and keen judgment enabled them to make many shrewd
guesses, which have since been verified, but they neverthe-
less failed in their main object, viz., the discovery of some
method for establishing the truth of what they only surmised.
Their disappointment we may still find expressed in such
names as dolerite (deceptive), aphanite (not apparent), etc.

What the earlier geologists, especially in Germany and
France, so earnestly desired, it was reserved for the students
of microscopical petrography to accomplish. Not even to-
day should crystalline rocks be studied altogether with the
microscope, but still the revival of interest in rock-study is
so entirely due to the introduction of this instrument, that



8 MODERN PETROGRAPHY.

modern petrography may properly be designated as micro-
scopical petrography. Other methods chemical, physical,
and geological are indispensable, but it is doubtful
whether these would have reached their present state of
development if it had not been for the new impetus, first
given to this department of inquiry by the application to it
of the microscope.

In his admirable historical review of the introduction of
the microscope into the study of mineralogy and geology,
Professor Zirkel mentions many attempts, some of them
nearly as old as the discovery of the instrument itself, to
apply it to the investigation of natural inorganic substances.
Minerals were microscopically examined as early as 1663.
At the beginning of the present century, Fleurian de Belle-
vue and Cordier studied rock-powder in the same way, but
without marked success. Sir David Brewster early used
polarized light in his investigations of the inclusions in
natural crystals ; while in the year 1830, William Nicol, the
inventor of the invaluable calcite prism which bears his
name, even prepared thin sections of petrified wood for
microscopic study. In 1834, Talbot contrived a microscope
provided with two nicol prisms for the easy production of
polarized light ; but, notwithstanding the discovery of all
these instruments and methods, it occurred to no one how
useful they could be made to geology by their application to
the systematic study of rocks. The microscopical examina-
tion of mineral powders and splinters in reflected light nat-
urally yielded no satisfactory results ; and it was not until
1850 that H. Clifton Sorby, Esq., of Sheffield, Eng., ex-
amined the first rock-section in transmitted light. But
despite this beginning, the sporadic attempts in this direc-
tion during the next twelve years,, both in England and on
the continent, profited but little. The real possibilities of
microscopical petrography were not in the faintest degree



MODERN PETROGRAPHY. 9

realized. The microscope was regarded merely as an amus-
ing toy ; the main effort was to discover with it something
novel and curious, not to apply it scientifically to the solu-
tion of broad geological problems. Even Sorby's papers,
which continued to be most suggestive in this line of work,
had reference only to very special points ; and it may be
doubted if his greatest service was not the transplanting of
his ideas and methods to Germany, where they were des-
tined to rapidly take root, and bear a fruitful harvest. Pro-
fessor Fouque'* has so well described the circumstances
under which this came about, that Professor Zirkel himself
has seen fit to quote his words ; and we cannot do better
than follow his example : " En 1862, il (Sorby) avait entrepris
avec sa mere un voyage d'agrement sur les bords du Rhin.
Arrive' a Bonn, il fit connaissance d'un e'leve du corps des
mines de Prusse, nomme Zirkel, par lequel il fut accompagne
et dirige dans quelques excursions. Us visiterent ensemble
1'Eifel, le Siebengebirge, et les environs du lac de Laach.
Chaque jour, chemin faisant, une conversation inte'ressante
et anirne'e s'engageait entre le touriste et son guide sur
la nature des roches volcaniques, sur les mineraux qui
les composent, et sur les merveilleux details de struc-
ture que le microscope y re've'le. Sorby exposait avec
clarte et chaleur les magnifiques re'sultats de ses eludes.
Le soir, apres 1'excursion de la journe'e, 1'entretien se
prolongeait encore. Enfin, de retour a Bonn, le maitre
improvise mit sous les yeux de son jeune auditeur quel-
ques preparations microscopiques qu'il avait apporte'es, et
lui fit appre'cier par lui-meme la nettete' et Timportance
des faits qui avaient e'te 1'objet de leurs longues causeries.
Quelques jours plus tard, en quittant Zirkel, il laissait
en lui un disciple enthousiaste, qui, desormais se consa-
crant entierement aux etudes de ge'ologie micrographique,
* Revue des deux mondes, July 15, 1879, P- 49



IO MODERN PETROGRAPHY.

allait bientot dans cette voie marcher de decouvertes en
decouvertes, grouper autour de lui un essaim de travail-
leurs, et devenir Tun des savans les plus celebres de
I'Allemagne."

Incited by the enthusiasm of his friend, and fully realiz-
ing the great importance of the microscopical study of rocks
to geology, Zirkel undertook, during the winter of 1862-63, in
the laboratory of the geological Reichsanstalt at Vienna, the
first systematic study of rock-sections as an end in itself.
Heretofore, all such investigations had been accidental, or, at
most, accessory to some other result which was aimed at.
The difficulties in the way were, however, very great ; and at
first, as is usually the case in an entirely new departure in
scientific research, but little interest in or sympathy with the
work was manifested. The minerals in transmitted light
under the microscope exhibited an altogether different char-
acter from that which they ordinarily presented when
macroscopically examined ; and the task of gradually recog-
nizing them in their new guise was a slow and discouraging
one. Nevertheless, energy and patience overcame the
obstacles. Mistakes were constantly made, but as con-
stantly corrected, until, at length, an amount of experience
was accumulated, which fixed with tolerable accuracy the
microscopical characteristics of the commonest rock-forming
minerals. During the nine years following the beginning of
his investigations, Zirkel published a series of important
articles in the Sitzungsberichte der Wiener Akademie, in
PoggendorfFs Annalen, in the Zeitschrift der deutschen
geologischen Gesellschaft, and in the Neues Jahrbuch fur
Mineralogie, Geologic, and Palaeontologie, which disclosed
the progress made in the microscopical identification of min-
erals, as well as numerous observations regarding the
impurity of apparently homogeneous minerals, whose in-
clusions and structure had never before been suspected.



MODERN PETROGRAPHY. II

Nor was it long before another important result of micro-
scopical rock-study was attained, viz., the discovery that
several mineral species, like leucite, nepheline, apatite,
sphene, tridimite, etc., generally considered very rare, pos-
sessed, as microscopic rock-constituents, a wide distribution.
This was made especially apparent in Zirkel's Basalt-
gesteine, published in 1870, in which he found it necessary
to add to the ordinary feldspar-basalts two other classes,
characterized by their containing leucite or nepheline as
an essential constituent.

But it must not be imagined that during all these years
Zirkel was alone in his microscopical studies of rocks. In
1864 appeared Laspeyres' investigations of the porphyries
near Halle ; and soon after, many other similar papers on
volcanic rocks, by Vom Rath, Kosmann, Weiss, Dressel,
and others. Fischer's Critical Micromineralogical Studies
(1869-1873) gave the results of a large number of observa-
tions, which showed how few of the so-called mineral species
possessed really homogeneous crystals. This accounted in a
satisfactory manner for the frequent discrepancies in analy-
ses of the same mineral from different localities, or by differ-
ent investigators. But perhaps the man whose work during
this first period in the development of microscopical petrog-
raphy (1862-1873) gave the most promise was Hermann
Vogelsang. He it was who seemed earliest and best to
realize what important services the microscope was capa-
ble of rendering to geology. In 1867, his Philosophy
of Geology was published, in which the third division,
entitled " Modern Geology," was devoted to microscopical
observations. This is even at the present time a most valu-
able and useful book. It is written in a charming style, is
full of most suggestive ideas, the true force of many of
which is only now commencing to be thoroughly appreciated,
while its colored plates, whose accuracy and beauty have



12 MODERN PETROGRAPHY.

never yet been excelled, make it still one of the most
important volumes in a petrographical library. Nor was
Vogelsang content alone to observe. He was a born experi-
menter, and was constantly striving to artificially reproduce
the results of nature under circumstances similar to hers,
where the processes could be studied. In this he was
very successful. In 1868, he and Geissler proved for the
first time the presence of liquid carbon-dioxide in quartz.
Sir David Brewster and Sorby had long before suspected it;
but Vogelsang was able to extract the liquid, and examine
its spectrum. Later, he devoted his attention to the study
of the devitrification products formed in glassy rocks, compar-
ing them with similar forms in artificial slags, and finally
reproducing their various characteristic shapes, by allowing
sulphur to crystallize under the microscope, where the forma-
tion and growth of the crystals were retarded by Canada
balsam. The results of these most interesting studies and
experiments were, alas ! destined to be given to the world
only after their talented author had quitted it, a fact which
cannot be too deeply deplored, as it deprived the young
science of one of its most earnest and successful cultivators.
Vogelsang's last work, edited by Professor Zirkel, and pub-
lished at Bonn, in 1875, under the title Die Krystalliten,
will always remain a monument to his carefulness and
originality.

In the year 1873 microscopical petrography entered upon
a new period in its development. The new science, which
at first was regarded as something curious and novel rather
than as anything really useful, gradually commenced to make
itself felt. Older geologists began to realize the important
role it was destined to play, while the younger workers were
attracted to it as the newest and least occupied field of dis-
covery. Still, any thorough knowledge of the young depart-
ment was confined to the few pioneers who had themselves



MODERN PETROGRAPHY. 13

shaped its growth. The need of a reliable text-book, which
should contain in systematic form all the information which
had been gathered regarding the identification of rock-form-
ing minerals under the microscope, was constantly becoming
more pressing, and the year 1873 witnessed the production
of two. One of these, by Professor Zirkel, entitled Die
mikroskopische Beschaffenheit der Mineralien und Gesteine^
brought together in available shape all the results thus far
attained, which were widely scattered through numerous
periodicals. The other, by Prof. Heinrich Rosenbusch,
called Die mikroskopische Physiographie der petrographisch
wichtigen Mineralien, gave new and more scientific methods
for the recognition and diagnosis of minerals in rock-sections
than had ever before been employed. Rosenbusch, who, with
his inaugural dissertation on the nephelinite of the Katzen-
buckel, had only a short time previously appeared as a
worker in the ranks of petrography, was destined very soon
to become recognized as its leader. The investigation of
the optical properties of crystals had long occupied the atten-
tion of eminent mineralogists. Des Cloizeaux had studied
their behavior in converged polarized light ; Von Kobell, by
his ingenious invention of the stauroscope, had discovered
a means of readily determining the position of the axes of
elasticity relative to the crystallographic axes ; Haidinger
had especially examined the phenomenon of pleochroism,
while Tschermak had published comparative optical studies
of very important groups of rock-forming minerals, like the
feldspars and amphibole-pyroxene family. It is Rosenbusch's
great service to petrography to have been the first to show
how these various optical methods, so carefully elaborated
for individual crystals when cut in known directions, could be
applied to their identification when occurring in confused
aggregates and intersected in every possible direction. The
microscope which he described as especially fitted for petro-



14 MODERN PETROGRAPHY.

graphical work,* was provided with a revolving stage, a con-
venient polarizing apparatus, and lenses for securing either
converged or parallel light at will. It still remains essen-
tially unchanged as the most convenient model. Rosen-
busch's method was accurate, and in nearly every case
sufficient. It has been amplified and improved in many
respects, but it remains to-day the basis upon which all the
truly scientific claims of microscopical petrography must rest.
Before 1873 all mineralogical determinations in fine-grained
rocks had been hazardous and empirical; it was only the
logical development and application of the established
principles of optical mineralogy to the study of rock-
sections which could place this on a firm and established
footing.

After the publication of the two above-named text-books
in 1873, the position of petrography as a department of
science, and even as an educational discipline, was not only
abundantly assured, but its growth was exceedingly rapid.
Other text-books, like those of Von Lasaulx, Lang, and
Fouque' and Michel-Le'vy, followed each other in quick suc-
cession. As Vogelsang's keen insight had discerned as