This chapter is intended to present a brief sketch of the progress of
the science of petrology from its early beginnings down to the present
time. The field to be covered is so large that this can be done only in
broadest outline, and it has therefore been restricted chiefly to what
has been accomplished in America. Although the period covered by the
life of the Journal extends backward for a century it is, however,
practically only within the last fifty years that the rocks of the
earth’s crust have been made the subject of such systematic
investigation by minute and delicately accurate methods of research as
to give rise to a distinct branch of geologic science. It is not
intended of course to affirm by this statement that the broader features
of the rocks, especially those which may be observed in the field and
which concern their relations as geologic masses, had not been made the
object of inquiry before this time, since this is the very foundation of
geology itself. Moreover, a certain amount of investigation of rocks, as
to the minerals of which they were composed, the significance of their
textures, and their chemical composition, had been carried out,
concomitant with the growth from early times of geology and mineralogy.
Thus, in 1815, Cordier by a process of washing separated the components
of a basalt and by chemical tests determined the constituent minerals.
At the time the Journal was founded, and for many years following, the
genesis of rocks, especially of igneous rocks, was a subject of inquiry
and of prolonged discussion. The aid of the rapidly growing science of
chemistry was invoked by the geologists and analyses of rocks were made
in the attempt to throw light on important questions. It is remarkable,
also, how keen were the observations that the geologists of those days
made upon the rocks, as to their component minerals and structures,
aided only by the pocket lens. Many ideas were put forward, the
essentials of which have persisted to the present day and have become
interwoven into the science, whereas others gave rise to contentions
which have not yet been settled to the satisfaction of all. At times in
these earlier days the microscope was called into use to help in solving
questions regarding the finer grained rocks, but this employment, as
Zirkel has shown, was merely incidental, and no definite technique or
purpose for the instrument was established.
On the other hand, the fact that up to the middle of the last century a
large store of information relating to the occurrence of rocks, and to
the mineral composition of those of coarser grain, and somewhat in
respect to their structure, had been accumulated, caused attempts in one
way or another to find means of coördinating these data and to produce
classifications, such as those of Von Cotta and Cordier. The history of
these attempts at classification, before the revelations made by the use
of the microscope had become general, has been admirably reviewed by
Whitman Cross[107] and need not be further enlarged upon here.
That a considerable amount of work was done along chemical lines also is
testified to by the publication of Roth’s Tabellen in 1861, in which all
published analyses of rocks up to that date were collected. What was
accomplished during this period was done chiefly on the continent of
Europe, and little attention had been paid to the subject of rocks
either in America or in Great Britain—even so late as 1870 Geikie
remarks, as referred to by Cross,[108] that there was no good English
treatise on petrography, or the classification and description of rocks.
In this country still less had been accomplished, interest being almost
wholly confined to the vigorous and growing sciences of geology and
mineralogy. This was natural, for mineralogy is the chief buttress on
which the structure of petrology rests and must naturally develop first,
especially in a relatively new and unexplored region, whose mineral
resources first attract attention. The geologists in carrying out their
studies also observed the rocks as they saw them in the field and made
incidental reference to them, but investigations of the rocks themselves
was very little attempted. An inspection of the first two series of the
Journal shows relatively little of importance in petrology published in
this country; a few analyses of rocks, occasional mention of mineral
composition, of weathering properties, and notices of methods of
classification proposed by French and German geologists nearly exhaust
the list.
_Introduction of the Microscope._
The beginnings of a particular branch of science are generally obscure
and rooted so imperceptibly in the foundations on which it rests that it
is difficult to point to any particular place in its development and say
that this is the start. There are exceptions of course, like the
remarkable work of Willard Gibbs in physical chemistry, and it may
chance that the happy inspiration of a single worker may give such
direction to methods of investigation as to open the gates into a whole
new realm of research, and to thus create a separate scientific field,
as happened in Radiochemistry.
This is what occurred in petrology when Sorby in England, in 1858,[109]
pointed out the value of the microscope as an instrument of research in
geologic investigations, and demonstrated that its employment in the
study of thin sections of rocks would yield information of the highest
value. Others beside Sorby had made use of the microscope, as pointed
out by Zirkel,[110] but, as he indicates, no one before him had
recognized its value. During the next ten years or so, however, its
recognition was very slow and the papers published by Sorby himself were
mainly concerned in settling very special matters.
As Williams[111] has suggested, the greatest service of Sorby was,
perhaps, his instructing Zirkel in his ideas and methods, for the latter
threw himself whole-heartedly into the study of rocks by the aid of the
microscope and his discoveries stimulated other workers in this field in
Germany, his native country, until the dawning science of petrology
began to assume form. A further step forward was taken in 1873 in the
appearance of the text-books of Zirkel[112] and Rosenbusch[113] which
collated the knowledge which had been gained and furnished the
investigator more precise methods of work. It is difficult for the
student of to-day to realize how much had been learned in the interval
and, for that matter, how much has been gained since 1873, without an
inspection of these now obsolete texts. In 1863, Zirkel, who was then at
the beginning of his work, said in his first paper presented to the
Vienna Academy of Sciences[114] that if he confined himself chiefly to
the structure of the rocks investigated and of their component minerals,
and stated little as to what these minerals were, the reason for that
was because “although the microscope serves splendidly for the
investigation of the former relations, it promises very little help for
the latter. Labradorite, oligoclase and orthoclase, augite and
hornblende, minerals whose recognition offers the most important
problems in petrography, in most cases cannot be distinguished from one
another under the microscope.” How little could Zirkel have foreseen, at
this time, less than forty years later, that not only could labradorite
be accurately determined in a rock-section, but that in a few minutes by
the making of two or three measurements on a properly selected section,
its chemical composition and the crystallographic orientation of the
section itself could be determined!
_The Thin Section._
Before going further we may pause here a moment to consider the origin
and development of the thin section, without which no progress could
have been made in this field of research. When we reflect upon the
matter, it seems a marvelous thing indeed that the densest, blackest
rock can be made to yield a section of the ¹⁄₁₀₀₀ of an inch in
thickness, so thin and transparent that fine printing can be easily read
through it, and transmitting light so clearly that the most high-powered
objectives of the microscope can be used to discern and study the
minutest structures it presents with the same capacity that they can be
employed upon sections of organic material prepared by the microtome.
This is no small achievement.
The first thin sections appear to have been prepared in 1828 by William
Nicol of Edinburgh, to whom we owe the prism which carries his name. He
undertook the making of sections from fossil wood for the purpose of
studying its structure. The method he developed was in principle the
same as that employed to-day, where machinery is not used; that is, he
ground a flat smooth surface upon one side of a chip of his petrified
wood, then cemented this to a bit of glass plate with Canada balsam, and
ground down the other side until the section was sufficiently thin. This
method was used by others for the study of fossil woods, coal, etc., but
it was not applied to rocks until 1850, when Sorby used it for
investigating a calcareous grit. Oschatz, in Germany, also about this
time independently discovered the same method. A further advance was
made in melting the cement, floating off the slice, and transferring it
to a suitable object-glass with cover, a process still employed by many;
though most operators now cement the first prepared surface of the rock
chip directly to the object-glass, and mount the section without
transferring it.
Next came the use of machinery to save labor in grinding, and another
step was made in the introduction of the saw, a circular disk of sheet
iron whose edge was furnished with embedded diamond dust. This makes it
possible to cut relatively thin slices with comparative rapidity, but
the final grinding which requires experience and skill must still be
done by hand. Carborundum has also largely replaced emery. The skill and
technique of preparers has reached a point where sections of rocks of
the desired thinness (0·001 inch), and four or five inches square have
been exhibited.
_The Era of Petrography._
In these earlier days of the science, as noted above, great difficulty
was at first experienced in the recognition of the minerals as they were
encountered in the study of rocks under the microscope. At that time the
chemical composition and outward crystal form of minerals were
relatively much better known than their physical and, especially, their
optical properties and constants. Some beginnings in this had been made
by Brewster, Nicol, and other physicists, and the mineralogists had
commenced to study minerals from this viewpoint. Especially Des
Cloiseaux had devoted himself to determining the optical properties of
many minerals, and the writer, when a student in the laboratory of
Rosenbusch in 1890, well recalls the tribute that he paid to the work of
Des Cloiseaux for the aid which it had afforded him in his earlier
researches in petrography.
The twenty years following the publication of the texts of Rosenbusch
and Zirkel may be characterized as the era of microscopical petrography.
A distinction is drawn here between the latter word and petrology, a
distinction often overlooked, for _petrography_ means literally the
description of rocks, whereas _petrology_ denotes the science of rocks.
As time passed the broader and more fundamental features of rocks,
especially of igneous and metamorphic rocks, in addition to their
mineral constitution, were more studied and gained greater recognition,
petrography gradually became a department of the larger field of
petrology—the science of to-day.
The use of the microscope, as soon as the method became more generally
understood, opened up so vast a field for investigation that at first
the study and description of the rocks seemed of prime importance. This
was natural, for hitherto the finer grained rocks had for the most part
defied any adequate elucidation and here was a key which enabled one to
read the cipher. A flood of literature upon the composition, structure,
and other characters of rocks from all parts of the world began to
appear in ever increasing volume. The demands of the petrographers for a
greater and more accurate knowledge of the physical and optical
constants of minerals stimulated this side of mineralogy, and increasing
attention was given to investigations in this direction. No definite
line between the two closely related sciences could be drawn, and a
large part of the work published under the heading of petrography could
perhaps be as well, or better, described under the title of
micro-mineralogy. To some, in truth, the rocks presented themselves
simply as aggregates of minerals, occurring in fine grains.
The work of the German petrographers attracted attention and drew
students from all parts of the world to their laboratories, especially
to those of Zirkel and Rosenbusch. The great opportunities, facilities,
and freedom for work which the German universities had long offered to
foreign students of science naturally encouraged this. In France a
brilliant school of petrologists, under the able leadership of
Michel-Lévy and Fouqué, had arisen whose work has been continued by
Barrois, Lacroix and others, but the rigid structure of the French
universities at that period did not permit of the offering of great
inducements for the attendance of foreign students. The work of the
French petrographers will be noticed in another connection.
In Great Britain, the home of Sorby, the new science progressed at first
slowly, until it was taken up by Allport, Bonney, Judd, Rutley, and
others. In 1885 the evidence of the advance that had been made and of
the firm basis on which the new science was now placed appeared in
Teall’s great work, “British Petrography,” which marked an epoch in that
country in petrographic publication. This work was of importance also in
another direction than that of descriptive petrography, in that it
contains valuable suggestions for the application of the principles of
modern physical chemistry in solving the problems of the origin of
igneous rocks. In it, as in the publications of Lagorio, we see the
passage of the petrographic into the petrologic phase of the science.
The earliest publication in America of the results of microscopic
investigation of rocks that the writer has been able to find is by A. A.
Julien and C. E. Wright, chiefly on greenstones and chloritic schists
from the iron-bearing regions of upper Michigan.[115] Naturally, it was
of a brief and elementary character. In 1874 E. S. Dana read a paper
before the American Association for the Advancement of Science on the
result of his studies on the “Trap-rocks of the Connecticut valley,” an
abstract of which was published in this Journal.[116] Meanwhile Clarence
King, in charge of the 40th Parallel survey, feeling the need of a
systematic study of the crystalline rocks which had been encountered,
and finding no one in this country prepared to undertake it, had induced
Zirkel to give his attention to this task. The result of this labor
appeared in 1876 in a fine volume[117] which attracted great attention.
In the same year appeared also petrographical papers by J. H.
Caswell,[118] E. S. Dana[119] and G. W. Hawes.[120] The latter devoted
himself almost entirely to this field of research and may thus, perhaps,
be termed the earliest of the petrographers in this country. His work,
“The Mineralogy and Lithology of New Hampshire,” issued in 1878 as one
of the reports of the State Survey under Prof. C. H. Hitchcock, was the
first considerable memoir by an American. This was followed by various
papers, one on the “Albany Granite and its contact phenomena,”[121]
being of especial interest as one of the earliest studies of a contact
zone, and in the fullness of methods employed in attacking the problem
forecasting the change to the petrology era.
During the ten years following, or from 1880 to 1890, the new science of
petrography flourished and grew exceedingly. Many young geologists
abroad devoted themselves to this field of research and the store of
accumulated knowledge concerning rocks from all parts of the world, and
their relations grew apace. The work of Teall has been noticed and among
others might be mentioned the name of Brögger, whose first
contribution[122] in this field gave evidence that his publications
would become classics in the science.
In America there appeared in this period a number of eager workers,
trained in part in the laboratories of Rosenbusch and Zirkel, whose
researches were destined to place the science on the secure footing in
this country which it occupies to-day. Among the earlier of these may be
mentioned Whitman Cross, R. D. Irving, J. P. Iddings, G. H. Williams, J.
F. Kemp, J. S. Diller, B. K. Emerson, M. E. Wadsworth, G. P. Merrill, N.
H. Winchell, and F. D. Adams in Canada. Others were added yearly to this
group. As a result of their work a constantly growing volume of
information about the rocks of America became available, and one has
only to examine the files of the Journal and other periodicals and the
listed publications of the National and State Surveys to appreciate
this.
In the Journal, for example, we may refer to papers[123] by Emerson on
the Deerfield dike and its minerals, and on the occurrence of nephelite
syenite at Beemersville, N. J.; to various interesting articles by Cross
on lavas from Colorado and the pneumatolytic and other minerals
associated with them; to important papers by Iddings on the rocks of the
volcanoes of the Northwest, and those of the Great Basin, to primary
quartz in basalt, and the origin of lithophysæ; to the results of
researches by G. H. Williams on the rocks of the Cortlandt series, and
on peridotite near Syracuse, N. Y.; to papers by Diller on the
peridotites of Kentucky, and recent volcanic eruptions in California; to
articles by R. D. Irving on the copper-bearing and other rocks of the
Lake Superior region, and to Kemp on dikes and other eruptives in
southern New York and northern New Jersey. Other publications would
greatly extend this list.
_The Petrologic Era._
As the chief facts regarding rocks, especially igneous rocks, as to
their mineral and chemical composition, their structure and texture and
the limits within which these are enclosed, became better known; and the
relations, which these bear to the associations of rocks and their modes
of occurrence, began to be perceived, the science assumed a broader
aspect. The perception that rocks were no longer to be regarded merely
as interesting assemblages of minerals, but as entities whose characters
and associations had a meaning, increased. More and better rock analyses
stimulated interest on the chemical side and this and the genesis of
their minerals led to a consideration of the magmas and their functions
in rock-making. The fact that the different kinds of rocks were not
scattered indiscriminately, but that different regions exhibited certain
groupings with common characters, was noticed. These features led to
attempts to classify igneous rocks on different lines from those
hitherto employed, and to account for their origin on broad principles.
In other words, the descriptive science of petrography merged into the
broader one of petrology. No exact time can be set which marks this
passage, since the evolution was gradual. Yet for this country, in
reviewing the literature, for which the successive issues of the
“Bibliography of North American Geology” published by the U. S.
Geological Survey has been of the greatest value; the writer has been
struck by the fact that in the first volume containing the index of
papers down to and including 1891, the articles on subjects of this
nature are listed under the heading of _petrography_, whereas in the
second volume (1892–1900) they are grouped under _petrology_ and the
former heading is omitted. A justification for this is found in
examining the list of publications and noting their character. With some
reason, therefore, the beginning of this period may be placed as in the
early years of this decade. Furthermore, it was at this time that the
great work of Zirkel[124] began to appear, which sums up so completely
the results of the petrographic era. Rosenbusch[125] was formulating
more definitely his views on the division of rocks into magmatic groups,
as displayed by their associations in the field, and using this in
classification; an idea which, appearing first in the second edition of
his “Physiographie der massigen Gesteine,” finds fuller development in
the third and last editions of this work. In this country Iddings[126]
published an important paper, in which the family relationships of
igneous rocks and the derivation of diverse groups from a common magma
by differentiation are clearly brought out. The fundamental problems
underlying the genesis of igneous rocks had now been clearly recognized,
and with this recognition the science passed into the petrologic phase.
Brögger[127] also had ascribed to the alkalic rocks of South Norway a
common parentage and had pointed out their regional peculiarities.
From this time forward an attempt may be noted to find an analogy
between rocks and the forms of organic life and to apply those
principles of evolution and descent, which have proved so fruitful in
the advancement of the biological sciences, to the genesis and
classification of igneous rocks. This, perhaps, has on the whole been
more apparent than real, in the constant borrowing of terms from those
sciences to express certain features and relationships observed, or
imagined, to obtain among rocks. Nevertheless, the perception of certain
relations which we owe so largely to Rosenbusch and to Brögger[128] has
proved of undoubted value in furnishing a stimulus for the investigation
of new regions, and in affording indications of what the petrologist
should anticipate in his work.
Thus, the labors of the men previously mentioned, with those of Bayley,
Bascom, Cushing, Daly, Lane, Lawson, Lindgren, Pirsson, J. F. Williams,
Washington, and others, have thrown a flood of light upon the igneous
rocks of this continent, and has made it possible to draw many broad
generalizations concerning their origin and distribution. Thus, the
differentiated laccoliths of Montana[129] have been of service in
affording clear examples of the process of local differentiation. Many
papers published in the Journal during the last twenty years show this
evolution and growth of petrological ideas. The contributions from
American sources during this later period, and of which those in the
Journal form a considerable fraction, have indeed been of great weight
in shaping the development and future of the science.
By referring to the files of the Journal, it will be seen that they
cover a continually widening range of subjects concerning rocks, and
articles of theoretical interest are more and more in evidence, along
with those of a purely descriptive character.[130] Thus we find
discussions by Becker on the physical constants of rocks, on fractional
crystallization, and on differentiation; by Cross on classification; by
Adams on the physical properties of rocks; by Daly on the methods of
igneous intrusion; by Wright on schistosity; by Fenner on the
crystallization of basaltic magma; by Bowen on differentiation by
crystallization; by the writer on complementary rocks and on the origin
of phenocrysts; by Smyth on the origin of alkalic rocks; by Murgoci on
the genesis of riebeckite rocks; and by Barrell on contact-metamorphism.
These may serve as examples, selected almost at random, from the files
of the Journal, and we find with them articles descriptive of the
petrology of many particular regions, which often contain also matter of
general interest and importance, such as papers by Lindgren on the
granodiorite and related rocks of the Sierra Nevada; by Ransome on
latite; by Cross on the Leucite Hills; by Hague on the lavas of the
Yellowstone Park; by Pogue on ancient volcanic rocks from North
Carolina; by Warren on peridotites from Cumberland, R. I.; on sandstone
from Texas by Goldman; and on the petrology of various localities in
central New Hampshire by Washington and the writer. Such a list could of
course be much extended and other papers of importance be cited, but
enough has been said to indicate how important a repository of the
results of petrologic research the Journal has been and continues to be.
In thus looking backward over the list of active workers we are
involuntarily led to pause and reflect how great a loss American
petrology has sustained in the premature death of some of its most
brilliant and promising exponents; it is only necessary to recall the
names of R. D. Irving, G. H. Williams, G. W. Hawes, J. F. Williams and
Carville Lewis, to appreciate this.
The store of material gathered during these years has led to the
publication of extensive memoirs, in which the science is treated not
from the older descriptive side, but from the theoretical standpoint and
of classification.[131] In these works strong divergencies of views and
opinions are observed, which is a healthy sign in a developing science.
It should be also noted that along with this evolution on the
theoretical side there has been a constant improvement in the technique
of investigating rocks. It is only necessary to compare the older
handbooks of Zirkel and Rosenbusch with the many modern treatises on
petrographic methods to be assured of this.[132] It is due on the one
hand to the vast amount of careful work which has been done in
accurately determining the physical constants of rock-minerals[133] and
in arranging these for their determination microscopically, as in the
remarkable studies on the feldspars by Michel-Lévy, and on the other in
researches on the apparatus employed, and in consequent improvements in
them and in ways of using them, as exemplified in the delicately
accurate methods introduced by Wright.[134] The development of the
microscope itself as an instrument of research in this field and in
mineralogy deserves a further word in this connection. The first step
toward making the ordinary microscope of special use in this way was
taken by Henry Fox Talbot of England, when he introduced in 1834 the
employment of the recently invented nicol prisms for testing objects in
polarized light. The modern instrument may be said to date from the
design offered by Rosenbusch in 1876. Since that time there have been
constant improvements, almost year by year, until the instrument has
become one of great precision and convenience, remarkably well adapted
for the work it is called upon to perform, with special designs for
various kinds of use, and an almost endless number of accessory
appliances for research in different branches of mineralogy and
crystallography, as well as in petrography proper.[135] This also calls
to mind the fact that for the convenience of those who are not able to
use the microscope special manuals of petrology have been prepared in
which rocks are treated from the megascopic standpoint.[136]
_Metamorphic Rocks._
In this connection the metamorphic rocks should not be forgotten. They
afford indeed the most difficult problems with which the geologist has
to deal; every branch of geological science may in turn be called upon
to furnish its quota for help in solving them. Under the attack of
careful, accurate and persistent work in the field, under the microscope
and in the chemical laboratory, with the aid of the garnered knowledge
in petrology, stratigraphy, physiography, and other fields of geologic
science, their mystery has in large part given way. The inaugural work
of Lehmann, Lossen, Barrois, Bonney, Teall, and other European
geologists, was paralleled in America by that of R. D. Irving, owing to
whose efforts the Lake Superior region became the chief place of study
of the metamorphic rocks in this country. Irving soon obtained the
assistance of G. H. Williams, who had been engaged in the study of such
rocks, and the latter published a memoir on the greenstone schist areas
of Menominee and Marquette in Michigan[137] which will always remain one
of the classics in the literature of metamorphic rocks. Irving’s own
contributions to petrology, though valuable, were cut short by his
untimely death, but the study of this region under the direction of his
associate and successor, C. R. Van Hise, with his co-laborers, has
yielded a mass of information of fundamental importance in our
understanding of metamorphism and the crystalline schists. Its fruitage
appears in the memoir by Van Hise[138] which is the authoritative work
of reference on metamorphism, and in various publications by him and his
assistants, Bayley, Clements, Leith, and others. The work of the
Canadian geologists, and of Kemp, Cushing, Smyth and Miller in the
Adirondack region, should also be mentioned in connection with this
field of petrology.
_Chemical Analyses of Rocks._
It has been previously pointed out that, as the science of petrology
grew, chemical investigations of rocks in bulk were undertaken. The
object of such analyses was to obtain on the one hand a better control
over the mineral composition and on the other to gain an idea of the
nature of the magmas from which igneous rocks had formed. The earliest
analysis of an American rock of which I can find record is of a “wacke”
by J. W. Webster given in the first volume of the Journal, page 296,
1818.
During the next 40 years a few occasional analyses were undertaken by
American chemists, by C. T. Jackson, T. Sterry Hunt, and others. In
1861, Justus Roth published the first edition of his Tabellen, in which
he included all analyses which had been made to that date and which he
considered were worthy of preservation. Although, naturally, from the
status of analytical chemistry up to that time, most of these would now
be considered rather crude, the publication of the work was of great
service and marked an epoch in geochemistry. In these tables Roth lists
four analyses of American igneous rocks, two from the Lake Superior
region by Jackson and J. D. Whitney and two by European chemists, one of
whom was Bunsen. The material of the last two was a “dolerite” and the
same locality is given for each—“Sierra Nevada between 38° and 41°”
which was probably considered quite precise for western America in those
days.
From these feeble beginnings the forward progress of petrology on the
chemical side in this country has been a steady one until its
development has reached the point which will be indicated in what
follows.
The collection of material by the various State surveys and by those
initiated by the National Government led to an increasing number of
rocks being analyzed during the petrographic period. These became also
increasingly good in quality, like those published by G. W. Hawes in his
papers. When, however, chemists were appointed to definite positions on
the staffs of the Government surveys and especially when, after the
organization of the U. S. Geological Survey in 1879, a general central
laboratory was founded in 1883 with F. W. Clarke in charge, then a new
era in the chemical investigation of rocks may be said to have started.
In this connection should be mentioned the work of W. F. Hillebrand, who
set a standard of accuracy and detail in rock analysis which had not
hitherto been attempted. As a consequence of his accurate and thorough
methods and results the mass of analyses performed by him and his fellow
chemists in this laboratory affords us the greatest single contribution
to chemical petrology which has been made. Up to January, 1914, the
report of Clarke[139] lists some 8000 analyses of various kinds made in
this laboratory for geologic purposes. Nearly everywhere also a great
improvement in the quality of rock-analyses is to be noted, and in the
manuals of Hillebrand[140] and Washington[141] the rock analyst has now
at his command the methods of a greatly perfected technique which should
insure him the best results.
Roth’s Tabellen have been previously mentioned; several supplements were
published, but after his death a long interval elapsed before this
convenient and useful work was again taken up by Washington[142] and
Osann.[143] A new edition of Washington’s Tables has recently been
published, listing some 8600 analyses of igneous rocks made up to the
close of 1913.[144]
On the theoretical side also, where petrology passes into geology, the
investigator of to-day will find a mass of most useful and accurate data
well discussed in the modern representative of Bischof’s Chemical
Geology—Clarke’s Data of Geochemistry.[145] The advance on the chemical
side, therefore, has been quite commensurate with that in the microscope
as an instrument, and in the results obtained by it.
_Physico-Chemical Work._
The study of geological results by experimental methods, which should
gain information concerning the processes by which those results are
caused, and the conditions under which they operate, has been from the
earliest days of the developing science recognized as most important,
and the record of the literature shows considerable was done in this
direction. Experimental work in modern petrology may, however, be
considered to date from 1882 when Fouqué and Michel-Lévy[146] published
the results of their extensive researches on the synthesis of minerals
and rocks by pyrogenous methods. The brilliant experiments of the French
petrologists at once attracted attention, and since that time a
considerable volume of valuable work has been done in this field by a
number of men, among whom may be mentioned Morozewicz,[147]
Doelter,[148] Tamman,[149] and Meunier.[150] As this work continued the
results of the rapid advances made in physical chemistry began to be
applied in this field with increasing value. To J. H. L. Vogt we owe a
valuable series of papers,[151] in which the formation of minerals and
rocks from magmas is treated from this standpoint. Most important of all
for the future of petrology has been the founding in Washington of the
splendid research institution, the Carnegie Geophysical Laboratory,
under the leadership of Dr. A. L. Day with its corps of trained
physicists, chemists and petrologists, devoted to the solving of the
problems which the progress of geological science raises. The
publications of this institution (many of them published in the Journal)
are too numerous to be mentioned here; many of them treat successfully
of matters of the greatest importance in petrology. This is an earnest
of what we may hope in the future. The accumulation of the exact
physical and chemical data, which is its aim, will serve as a necessary
check to hypothetical speculation and bring petrology, and especially
petrogenesis, in line with the other more exact sciences by furnishing
quantitative foundations for its structure of theory to rest upon.
While the achievements of this great organization seem to minimize the
work of the individual investigator in this field, he may take heart by
observing the important results on the strength of rocks under various
conditions which have been obtained by Adams in recent years, data of
wide application in theoretical geology. In this field also a special
text has appeared in which the principles and acquired data are
given.[152]
_Summary._
In this brief retrospect, giving only the barest outlines and omitting
from necessity much of importance, we have seen petrology grow from
occasional crude experiments into a fully organized science in the last
half century. It has to-day a well-perfected technique, a large volume
of literature, texts treating of general principles, of methods of work,
descriptive handbooks on the morphological side, and has attained
general recognition as a field, which, though not large, is worthy of
the concentration of intellectual endeavor. Like other healthy growing
organisms it has given rise to offshoots, and the sciences of
metallography and of the micro-study of ore deposits, which are rapidly
assuming form, have branched from it.
What of the future? The old days of mostly descriptive work, and of
theorizing purely from observed results, have passed. The science has
entered upon the stage where work and theory must be continually brought
into agreement with chemical, physical and mathematical laws and data,
and in the application of these new problems present themselves. As we
climb, in fact, new horizons open to our view indicating fresh regions
for exploration, for acquiring human knowledge and for our satisfaction.
_Bibliography._
Footnote 107:
W. Cross, Jour. Geology, =10=, 451, 1902.
Footnote 108:
_Ibid._, p. 45.
Footnote 109:
Sorby, Quart. Jour. Geol. Soc., =14=, 453, 1858.
Footnote 110:
Zirkel, Einführung des Mikroskops in das mineralogisch-geologische
Studium, 1881.
Footnote 111:
Williams, G. H., Modern Petrography, 1886.
Footnote 112:
Zirkel, Mikroskopische Beschaffenheit der Mineralien und Gesteine.
Footnote 113:
Rosenbusch, Mikroskopische Physiographie der petrographisch wichtigen
Mineralien.
Footnote 114:
Zirkel, Mikroskopische Gesteinstudien, Sitzung vom 12 März, 1863.
Footnote 115:
Julien and Wright, Geol. Surv. of Michigan, 2, 1873. Appendices A and
C.
Footnote 116:
Dana, E. S., the Journal, =8=, 390–392, 1874.
Footnote 117:
Zirkel, Geological Exploration of the 40th Parallel; vol. VI,
Microscopical Petrography.
Footnote 118:
Caswell, Microscopical Petrography of the Black Hills. U. S. Geog. and
Geol. Surv. Rocky Mts. Rep. on Black Hills of Dakota, 469–527. The
separate copies issued bear the imprint 1876; the complete report
1880.
Footnote 119:
Dana, E. S., Igneous Rocks in the Judith Mts. Rep. of Reconnaissance
Carroll, Mont., to Yellowstone Park in 1875. Col. Wm. Ludlow, War
Dept., Washington, 105–106.
Footnote 120:
Hawes, G. W., Rocks of the Chlorite Formation, etc., the Journal,
=11=, 122–126, 1876. Greenstones of New Hampshire, etc, ibid., =12=,
129–137, 1876.
Footnote 121:
Hawes, G. W., the Journal, =21=, 21–32, 1881.
Footnote 122:
Brögger, Die silurischen Etagen 2 und 3, Kristiania, 1882.
Footnote 123:
The references for the papers alluded to, all of them in the Journal,
are as follows:
Emerson, =24=, 195–202, 270–278, 349–359, 1882;
——, =23=, 302–308, 1882.
Cross, =27=, 94–96, 1884; =31=, 432–438, 1886; =39=, 359–370,
1890; =41=, 466–475, 1891; =23=, 452–458, 1882.
Iddings, =26=, 222–235, 1883;
——, =27=, 453–463, 1884;
——, =36=, 208–221, 1888;
——, =33=, 36–45, 1887.
Williams, =31=, 26–41, 1886; =33=, 135–144, 191–199, 1887; =35=,
433–448, 1888; =36=, 254–259, 1888.
——, =34=, 137–145, 1887.
Diller, =32=, 121–125, 1886; =37=, 219–220, 1889;
——, =33=, 45–50, 1887.
Irving (=26=, 27–32, 321–322, =27=, 130–134, 1883; =29=, 358–359,
1885).
Kemp (=35=, 331–332, 1888; =36=, 247–253, 1888; =38=, 130–134,
1889).
Footnote 124:
Zirkel, Lehrbuch der Petrographie, 2d ed., 1893.
Footnote 125:
Hunter and Rosenbusch, Ueber Monchiquit, etc., Min. petr. Mitth.,
=11=, 445, 1890. Rosenbusch, Ueber Structur und Class. der
Eruptivgesteine, ibid., =12=, 351, 1891.
Footnote 126:
Iddings, Origin of Igneous Rocks, Bull. Phil, Soc. Washington, =12=,
89–213, 1892.
Footnote 127:
Brögger, Mineralien der Syenit-pegmatit-gànge, etc., Zs. Kryst., =16=,
1890.
Footnote 128:
——, Basic Eruptive Rocks of Gran, Quart. Jour. Geol. Soc., =50=, 15,
1894; Grorudit-Tinguait-Serie, Vidensk. Skrift. 1 Math. nat. Kl., No.
4, 1894.
Footnote 129:
Weed and Pirsson, _e. g._ Shonkin Sag, the Journal, =12=, 1–17, 1901.
Footnote 130:
The references for the articles mentioned (all in the Journal) are as
follows:
Becker, =46=, 1893; =4=, 257, 1897; =3=, 21–40, 1897.
Cross, =39=, 657–661, 1915.
Adams, =22=, 95–123, 1906; =29=, 465–487, 1910.
Daly, =22=, 195–216, 1906; =26=, 17–50, 1908.
Wright, =22=, 224–230, 1906.
Fenner, =29=, 217–234, 1910.
Bowen, =39=, 175–191; =40=, 161–185, 1915.
Pirsson, =50=, 116–121, 1895; =7=, 271–280, 1899.
Smyth, =36=, 33–46, 1913.
Murgoci, =20=, 133–145, 1905.
Barrell, =13=, 279–296, 1902.
Lindgren, =3=, 301–314, 1897; =9=, 269–282, 1900.
Ransome, =5=, 355–375, 1898.
Cross, =4=, 115–141, 1897.
Hague, =1=, 445–457, 1896.
Pogue, =28=, 218–238, 1909.
Warren, =25=, 12–36, 1908.
Goldman, =39=, 261–288, 1915.
Washington and Pirsson, Belknap Mts., =20=, 344–353, 1905; =22=,
439–457, 493–515, 1906.
——, Red Hill, =23=, 257–276, 433–447, 1907.
——, Tripyramid Mt., =31=, 405–431, 1911.
Footnote 131:
Quantitative Classification of Igneous Rocks, Cross, Iddings, Pirsson
and Washington, Chicago, 1903.
Petrogenesis, C. Doelter, Braunschweig, 1906.
Igneous Rocks, vols. =1= and =2=, J. P. Iddings, New York, 1909 and
1913.
Problem of Volcanism, Iddings, New Haven, 1914.
Natural History of Igneous Rocks, Alfred Harker, London, 1909.
Igneous Rocks and their Origin, R. A. Daly, New York, 1914.
Footnote 132:
Among these may be mentioned:
Rosenbusch u. Wülfing, Physiog. der petrog. wicht. Min.,
Stuttgart, 1905.
Iddings, J. P., Rock-Minerals, 1st ed., New York, 1906.
Johannsen, A., Manual of Petrographic Methods, New York, 1914.
Winchell, N. H. and A. N., Elements of Optical Mineralogy, New
York, 1909.
Footnote 133:
We may mention here, for example, the work in mineralogy of Penfield,
noticed in the accompanying chapter on mineralogy. In addition to the
accurate determination of the composition and constants of many
minerals, some of which have importance from the petrographic
standpoint, we owe to him more than anyone the recognition of fluorine
and hydroxyl in a variety of species, and thereby the perception of
their pneumatolytic origin. His papers have been published almost
entirely in the Journal.
Footnote 134:
Wright, Methods of Petrographic-Microscopic Research, Carnegie Inst.,
Washington, 1911, and various papers; many in the Journal.
Footnote 135:
Conf. Wright’s work quoted above and the various manuals previously
mentioned.
Footnote 136:
Kemp, Hand-book of Rocks, 3d ed., New York, 1904. Pirsson, Rocks and
Rock-Minerals, New York, 1910.
Footnote 137:
Williams, G. H., U. S. Geol. Surv., Bull. =62=, Washington, 1890.
Footnote 138:
Van Hise, Treatise on Metamorphism, U. S. Geol. Surv., Monograph =17=.
Footnote 139:
F. W. Clarke, U. S. Geol. Surv., Bull. =591=, 1915.
Footnote 140:
Hillebrand, Analysis of Silicate and Carbonate Rocks, U. S. Geol.
Surv., Bull. =422=, 1910.
Footnote 141:
Washington, Chemical Analysis of Rocks, pp. 200, New York, 1910.
Footnote 142:
Id., Chemical Analyses of Igneous Rocks (1884–1900), U. S. Geol.
Surv., Prof. Paper, No. =14=, 1903.
Footnote 143:
Osann, Beitr. zu chem. Petrogr., II Teil. Anal. d. Eruptivgest.,
1884–1900, Stuttgart, 1905.
Footnote 144:
Washington, ibid., 2d ed., U. S. Geol. Surv., Prof. Paper =99=, pp.
1216, 1917.
Footnote 145:
Clarke, U. S. Geol. Surv., Bull. =616=, 1916.
Footnote 146:
Fouqué and Michel-Lévy, Synthese des Mineraux et des Roches, Paris,
1882.
Footnote 147:
Morozewicz, Exper. Untersuch. u. Bildung der Min. im Magma, Min. petr.
Mitt., =18=, 1898.
Footnote 148:
Doelter, Synthetische Studien, N. Jahrb. Min. 1897, =1=, 1–26. Allg.
chem. Mineralogie, etc.
Footnote 149:
Tamman, Krystallisieren und Schmelzen, 1903.
Footnote 150:
St. Meunier, Les Méthodes de Synthèse en Minéralogie, Paris, 1891.
Footnote 151:
Vogt, Mineralbildung in Smelzmassen, Christiania, 1892;
Silikatschmelzlösungen, =1= and =2=, 1903, 1904, and various other
papers, esp. in Min. petr. Mitt., vols. =24= and =25=, 1906.
Footnote 152:
H. E. Boeke, Grundlagen der physikalisch-chemischen Petrographie,
Berlin, 1915.
VIII
THE GROWTH OF MINERALOGY FROM 1818 TO 1918