CHRONOLOGICAL CLASSIFICATION OF ROCKS.
Aqueous, plutonic, volcanic, and metamorphic rocks, considered
chronologically--Lehman's division into primitive and
secondary--Werner's addition of a transition class--Neptunian
theory--Hutton on igneous origin of granite--How the name of primary
was still retained for granite--The term "transition," why faulty--The
adherence to the old chronological nomenclature retarded the progress
of geology--New hypothesis invented to reconcile the igneous origin of
granite to the notion of its high antiquity--Explanation of the
chronological nomenclature adopted in this work, so far as regards
primary, secondary, and tertiary periods.
In the first chapter it was stated that the four great classes of rocks,
the aqueous, the volcanic, the plutonic, and the metamorphic, would each
be considered not only in reference to their mineral characters, and
mode of origin, but also to their relative age. The preservation of the
shelves may have required, says Darwin, the quick growth of green turf
on a good soil; their abrupt cessation may mark the place where the soil
was barren, and when a green sward formed slowly. In regard to the
aqueous rocks, we have already seen that they are stratified, that some
are calcareous, others argillaceous or siliceous, some made up of sand,
others of pebbles; that some contain freshwater, others marine fossils,
and so forth; but the student has still to learn which rocks, exhibiting
some or all of these characters, have originated at one period of the
earth's history, and which at another.
To determine this point in reference to the fossiliferous formations is
more easy than in any other class, and it is therefore the most
convenient and natural method to begin by establishing a chronology for
these fossiliferous strata, and then to endeavour to refer to the same
divisions, the several groups of plutonic, volcanic, and metamorphic
rocks. This system of classification is not only recommended by its
greater clearness and facility of application, but is also best fitted
to strike the imagination by bringing into one view the past changes of
the inorganic world, and the contemporaneous revolutions of the organic
creation. For the sedimentary formations of successive periods are most
readily distinguished by the different species of fossil animals and
plants which they inclose, and of which one race after another has
flourished and then disappeared from the earth.
But before entering specially on the subdivisions of the aqueous rocks
arranged according to the order of time, it will be desirable to say a few
words on the chronology of rocks in general, although in doing so we shall
be unavoidably led to allude to some classes of phenomena which the
beginner must not yet expect fully to comprehend.
It was for many years a received opinion, that the formation of entire
families of rocks, such as the plutonic and those crystalline schists
spoken of in the first chapter as metamorphic, began and ended before any
members of the aqueous and volcanic orders were produced; and although this
idea has long been modified, and is nearly exploded, it will be necessary
to give some account of the ancient doctrine, in order that beginners may
understand whence many prevailing opinions, and some part of the
nomenclature of geology, still partially in use, was derived.
About the middle of the last century, Lehman, a German miner, proposed to
divide rocks into three classes, the first and oldest to be called
primitive, comprising the hypogene, or plutonic and metamorphic rocks; the
next to be termed secondary, comprehending the aqueous or fossiliferous
strata; and the remainder, or third class, corresponding to our alluvium,
ancient and modern, which he referred to "local floods, and the deluge of
Noah." In the primitive class, he said, such as granite and gneiss, there
are no organic remains, nor any signs of materials derived from the ruins
of pre-existing rocks. Their origin, therefore, may have been purely
chemical, antecedent to the creation of living beings, and probably coeval
with the birth of the world itself. The secondary formations, on the
contrary, which often contain sand, pebbles, and organic remains, must have
been mechanical deposits, produced after the planet had become the
habitation of animals and plants. This bold generalization, although
anticipated in some measure by Steno, a century before, in Italy, formed at
the time an important step in the progress of geology, and sketched out
correctly some of the leading divisions into which rocks may be separated.
About half a century later, Werner, so justly celebrated for his improved
methods of discriminating the mineralogical characters of rocks, attempted
to improve Lehman's classification, and with this view intercalated a
class, called by him "the transition formations," between the primitive and
secondary. Between these last he had discovered, in northern Germany, a
series of strata, which in their mineral peculiarities were of an
intermediate character, partaking in some degree of the crystalline nature
of micaceous schist and clay-slate, and yet exhibiting here and there signs
of a mechanical origin and organic remains. For this group, therefore,
forming a passage between Lehman's primitive and secondary rocks, the name
of _uebergang_ or transition was proposed. They consisted principally of
clay-slate and an argillaceous sandstone, called grauwacke, and partly of
calcareous beds. It happened in the district which Werner first
investigated, that both the primitive and transition strata were highly
inclined, while the beds of the newer fossiliferous rocks, the secondary of
Lehman, were horizontal. To these latter, therefore, he gave the name
_floetz_, or "a level floor;" and every deposit more modern than the chalk,
which was classed as the uppermost of the floetz series, was designated "the
overflowed land," an expression which may be regarded as equivalent to
alluvium, although under this appellation were confounded all the strata
afterwards called tertiary, of which Werner had scarcely any knowledge. As
the followers of Werner soon discovered that the inclined position of the
"transition beds," and the horizontality of the floetz, or newer
fossiliferous strata, were mere local accidents, they soon abandoned the
term floetz; and the four divisions of the Wernerian school were then named
primitive, transition, secondary, and alluvial.
As to the trappean rocks, although their igneous origin had been already
demonstrated by Arduino, Fortis, Faujas, and others, and especially by
Desmarest, they were all regarded by Werner as aqueous, and as mere
subordinate members of the secondary series.[91-A]
This theory of Werner's was called the "Neptunian," and for many years
enjoyed much popularity. It assumed that the globe had been at first
invested by an universal chaotic ocean, holding the materials of all rocks
in solution. From the waters of this ocean, granite, gneiss, and other
crystalline formations, were first precipitated; and afterwards, when the
waters were purged of these ingredients, and more nearly resembled those of
our actual seas, the transition strata were deposited. These were of a
mixed character, not purely chemical, because the waves and currents had
already begun to wear down solid land, and to give rise to pebbles, sand,
and mud; nor entirely without fossils, because a few of the first marine
animals had begun to exist. After this period, the secondary formations
were accumulated in waters resembling those of the present ocean, except at
certain intervals, when, from causes wholly unexplained, a partial
recurrence of the "chaotic fluid" took place, during which various trap
rocks, some highly crystalline, were formed. This arbitrary hypothesis
rejected all intervention of igneous agency, volcanos being regarded as
modern, partial, and superficial accidents, of trifling account among the
great causes which have modified the external structure of the globe.
Meanwhile Hutton, a contemporary of Werner, began to teach, in Scotland,
that granite as well as trap was of igneous origin, and had at various
periods intruded itself in a fluid state into different parts of the
earth's crust. He recognized and faithfully described many of the
phenomena of granitic veins, and the alterations produced by them on the
invaded strata, which will be treated of in the thirty-second chapter.
He, moreover, advanced the opinion, that the crystalline strata called
primitive had not been precipitated from a primaeval ocean, but were
sedimentary strata altered by heat. In his writings, therefore, and in
those of his illustrator, Playfair, we find the germ of that metamorphic
theory which has been already hinted at in the first chapter, and
which will be more fully expounded in the thirty-fourth and
thirty-fifth chapters.
At length, after much controversy, the doctrine of the igneous origin of
trap and granite made its way into general favour; but although it was, in
consequence, admitted that both granite and trap had been produced at many
successive periods, the term primitive or primary still continued to be
applied to the crystalline formations in general, whether stratified, like
gneiss, or unstratified, like granite. The pupil was told that granite was
a primary rock, but that some granites were newer than certain secondary
formations; and in conformity with the spirit of the ancient language, to
which the teacher was still determined to adhere, a desire was naturally
engendered of extenuating the importance of those more modern granites, the
true dates of which new observations were continually bringing to light.
A no less decided inclination was shown to persist in the use of the
term "transition," after it had been proved to be almost as faulty in
its original application as that of floetz. The name of transition, as
already stated, was first given by Werner, to designate a mineral
character, intermediate between the highly crystalline or metamorphic
state and that of an ordinary fossiliferous rock. But the term acquired
also from the first a chronological import, because it had been
appropriated to sedimentary formations, which, in the Hartz and other
parts of Germany, were more ancient than the oldest of the secondary
series, and were characterized by peculiar fossil zoophytes and shells.
When, therefore, geologists found in other districts stratified rocks
occupying the same position, and inclosing similar fossils, they gave to
them also the name of _transition_, according to rules which will be
explained in the next chapter; yet, in many cases, such rocks were found
not to exhibit the same mineral texture which Werner had called
transition. On the contrary, many of them were not more crystalline than
different members of the secondary class; while, on the other hand,
these last were sometimes found to assume a semi-crystalline and almost
metamorphic aspect, and thus, on lithological grounds, to deserve
equally the name of transition. So remarkably was this the case in the
Swiss Alps, that certain rocks, which had for years been regarded by
some of the most skilful disciples of Werner to be transition, were at
last acknowledged, when their relative position and fossils were better
understood, to belong to the newest of the secondary groups; nay, some
of them have actually been discovered to be members of the lower
tertiary series! If, under such circumstances, the name of transition
was retained, it is clear that it ought to have been applied without
reference to the age of strata, and simply as expressive of a mineral
peculiarity. The continued appropriation of the term to formations of a
given date, induced geologists to go on believing that the ancient
strata so designated bore a less resemblance to the secondary than is
really the case, and to imagine that these last never pass, as they
frequently do, into metamorphic rocks.
The poet Waller, when lamenting over the antiquated style of Chaucer,
complains that--
We write in sand, our language grows,
And, like the tide, our work o'erflows.
But the reverse is true in geology; for here it is our work which
continually outgrows the language. The tide of observation advances with
such speed that improvements in theory outrun the changes of nomenclature;
and the attempt to inculcate new truths by words invented to express a
different or opposite opinion, tends constantly, by the force of
association, to perpetuate error; so that dogmas renounced by the reason
still retain a strong hold upon the imagination.
In order to reconcile the old chronological views with the new doctrine of
the igneous origin of granite, the following hypothesis was substituted for
that of the Neptunists. Instead of beginning with an aqueous menstruum or
chaotic fluid, the materials of the present crust of the earth were
supposed to have been at first in a state of igneous fusion, until part of
the heat having been diffused into surrounding space, the surface of the
fluid consolidated, and formed a crust of granite. This covering of
crystalline stone, which afterwards grew thicker and thicker as it cooled,
was so hot, at first, that no water could exist upon it; but as the
refrigeration proceeded, the aqueous vapour in the atmosphere was
condensed, and, falling in rain, gave rise to the first _thermal ocean_. So
high was the temperature of this boiling sea, that no aquatic beings could
inhabit its waters, and its deposits were not only devoid of fossils, but,
like those of some hot springs, were highly crystalline. Hence the origin
of the primary or crystalline strata,--gneiss, mica-schist, and the rest.
Afterwards, when the granitic crust had been partially broken up, land
and mountains began to rise above the waters, and rains and torrents
ground down rock, so that sediment was spread over the bottom of the
seas. Yet the heat still remaining in the solid supporting substances
was sufficient to increase the chemical action exerted by the water,
although not so intense as to prevent the introduction and increase of
some living beings. During this state of things some of the residuary
mineral ingredients of the primaeval ocean were precipitated, and formed
deposits (the transition strata of Werner), half chemical and half
mechanical, and containing a few fossils.
By this new theory, which was in part a revival of the doctrine of
Leibnitz, published in 1680, on the igneous origin of the planet, the old
ideas respecting the priority of all crystalline rocks to the creation of
organic beings, were still preserved; and the mistaken notion that all the
semi-crystalline and partially fossiliferous rocks belonged to one period,
while all the earthy and uncrystalline formations originated at a
subsequent epoch, was also perpetuated.
It may or may not be true, as the great Leibnitz imagined, that the whole
planet was once in a state of liquefaction by heat; but there are certainly
no geological proofs that the granite which constitutes the foundation of
so much of the earth's crust was ever at once in a state of universal
fusion. On the contrary, all our evidence tends to show that the formation
of granite, like the deposition of the stratified rocks, has been
successive, and that different portions of granite have been in a melted
state at distinct and often distant periods. One mass was solid, and had
been fractured, before another body of granitic matter was injected into
it, or through it, in the form of veins. Some granites are more ancient
than any known fossiliferous rocks; others are of secondary; and some, such
as that of Mont Blanc and part of the central axis of the Alps, of tertiary
origin. In short, the universal fluidity of the crystalline foundations of
the earth's crust, can only be understood in the same sense as the
universality of the ancient ocean. All the land has been under water, but
not all at one time; so all the subterranean unstratified rocks to which
man can obtain access have been melted, but not simultaneously.
In the present work the four great classes of rocks, the aqueous, plutonic,
volcanic, and metamorphic, will form four parallel, or nearly parallel,
columns in one chronological table. They will be considered as four sets of
monuments relating to four contemporaneous, or nearly contemporaneous,
series of events. I shall endeavour, in a subsequent chapter on the
plutonic rocks, to explain the manner in which certain masses belonging to
each of the four classes of rocks may have originated simultaneously at
every geological period, and how the earth's crust may have been
continually remodelled, above and below, by aqueous and igneous causes,
from times indefinitely remote. In the same manner as aqueous and
fossiliferous strata are now formed in certain seas or lakes, while in
other places volcanic rocks break out at the surface, and are connected
with reservoirs of melted matter at vast depths in the bowels of the
earth,--so, at every era of the past, fossiliferous deposits and
superficial igneous rocks were in progress contemporaneously with others of
subterranean and plutonic origin, and some sedimentary strata were exposed
to heat and made to assume a crystalline or metamorphic structure.
It can by no means be taken for granted, that during all these changes the
solid crust of the earth has been increasing in thickness. It has been
shown, that so far as aqueous action is concerned, the gain by fresh
deposits, and the loss by denudation, must at each period have been equal
(see above, p. 68.); and in like manner, in the inferior portion of the
earth's crust, the acquisition of new crystalline rocks, at each successive
era, may merely have counter-balanced the loss sustained by the melting of
materials previously consolidated. As to the relative antiquity of the
crystalline foundations of the earth's crust, when compared to the
fossiliferous and volcanic rocks which they support, I have already stated,
in the first chapter, that to pronounce an opinion on this matter is as
difficult as at once to decide which of the two, whether the foundations or
superstructure of an ancient city built on wooden piles, may be the oldest.
We have seen that, to answer this question, we must first be prepared to
say whether the work of decay and restoration had gone on most rapidly
above or below, whether the average duration of the piles has exceeded that
of the stone buildings, or the contrary. So also in regard to the relative
age of the superior and inferior portions of the earth's crust; we cannot
hazard even a conjecture on this point, until we know whether, upon an
average, the power of water above, or that of heat below, is most
efficacious in giving new forms to solid matter.
After the observations which have now been made, the reader will
perceive that the term primary must either be entirely renounced, or, if
retained, must be differently defined, and not made to designate a set
of crystalline rocks, some of which are already ascertained to be newer
than all the secondary formations. In this work I shall follow most
nearly the method proposed by Mr. Boue, who has called all
_fossiliferous_ rocks older than the secondary by the name of primary.
To prevent confusion, however, I shall always speak of these, when they
are of the aqueous class, as the _primary fossiliferous_ formations,
because the word primary has hitherto been almost inseparably connected
with the idea of a non-fossiliferous rock.
If we can prove any plutonic, volcanic, or metamorphic rocks to be older
than the secondary formations, such rocks will also be primary, according
to this system. Mr. Boue having with great propriety excluded the
metamorphic rocks, _as a class_, from the primary formations, proposed to
call them all "crystalline schists."
As there are secondary fossiliferous strata, so we shall find that there
are plutonic, volcanic, and metamorphic rocks of contemporaneous origin,
which I shall also term secondary.
In the next chapter it will be shown that the strata above the chalk have
been called tertiary. If, therefore, we discover any volcanic, plutonic, or
metamorphic rocks, which have originated since the deposition of the chalk,
these also will rank as tertiary formations.
It may perhaps be suggested that some metamorphic strata, and some
granites, may be anterior in date to the oldest of the primary
fossiliferous rocks. This opinion is doubtless true, and will be discussed
in future chapters; but I may here observe, that when we arrange the four
classes of rocks in four parallel columns in one table of chronology, it is
by no means assumed that these columns are all of equal length; one may
begin at an earlier period than the rest, and another may come down to a
later point of time. In the small part of the globe hitherto examined, it
is hardly to be expected that we should have discovered either the oldest
or the newest members of each of the four classes of rocks. Thus, if there
be primary, secondary, and tertiary rocks of the aqueous or fossiliferous
class, and in like manner primary, secondary, and tertiary hypogene
formations, we may not be yet acquainted with the most ancient of the
primary fossiliferous beds, or with the newest of the hypogene.
FOOTNOTES:
[91-A] See Principles, vol. i. chap. iv.