Alexander von Humboldt.

A geognostical essay on the superposition of rocks, in both hemispheres [microform] online

. (page 1 of 30)
Online LibraryAlexander von HumboldtA geognostical essay on the superposition of rocks, in both hemispheres [microform] → online text (page 1 of 30)
Font size
QR-code for this ebook









Printed by A. & R. Spottiswoode,
New- Street- Square.












c ^^v


1 HE work which I now submit to the judgment
of geognosts includes nearly the whole of the sub-
ject of positive geognosy. If "I have accomplished
the end which I proposed, the phenomena of the
most remarkable superpositions of rocks in both
hemispheres, north and south of the equator, will
appear arranged in the order of their mutual rela-
tions. I cannot flatter myself with having suc-
ceeded in comprising within so narrow a space, so
great a variety of objects ; but I hope that my
work will contain two sources of interest ; that of
making known a considerable number of observa-
tions which had not hitherto been published, and
that of presenting some general views on the suc-
cession of those rocks which have been considered
as the terms of a simple or a periodical series.

The comparison of the rocks of the old world
with those of the Cordillera of the Andes, has been
deduced exclusively from my own researches. To
guard myself against the danger of first impres-
sions, and the errors which might arise from certain
prejudices, I have, within a few months past, read
over all the manuscripts which I had written dur-
ing my travels ; and I have compared the descrip-
tions with the sections and profiles of the mountains
which were drawn on the spot. After having con-
sidered the whole of their geognostic relations, I
have confined myself to those which appeared to
me the most certain or the most probable, and I




frankly state what still demands a more particular

Previous to the application of systematic names
to the formations of the Andes, of the Oronoco,
of the Amazon, or of New Spain, I have described
their various relations of position, of composition,
and of structure. This method, which I have con-
stantly followed, will enable the reader to decide
more easily on the degree of confidence which my
arrangements merit. If it be recollected, that be-
fore my travels in equinoctial America, scarcely any
rock in that country had been named, and that I
could not be guided in the study of superpositions
by any anterior observations, it will, I hope, appear
less surprising, should all my descriptions not be
found equally perfect. The articles which I have
devoted to the different formations are of unequal
length, according to the number of facts which I
have been able to state respecting them.

In this geognostical essay, as well as in my re-
searches on the isothermal lines, on the geography of
plants, and on the laws which have been observed
in the distribution of organic bodies, I have endea-
voured, at the same time that I presented the detail
of the phenomena, to generalize the ideas respect-
ing them, and to connect them with the great
questions in natural philosophy. I have dwelt
chiefly on the phenomena of alternation, of oscilla-
tion, and of local suppression, and in those which
result from the passage of one formation to another
in consequence of interior developement. These
subjects are not mere theoretical speculations ; far
from being useless, they lead us to the knowledge
of the laws of nature. It would degrade the sciences


to make their progress depend solely on the accu-
mulation and study of particular phenomena.

It is already many years since I first announced
the table of positions which I now publish. The
hesitation with which it is usual to proceed to the
printing of a work long expected, would perhaps
have still farther retarded this publication, had I
not been compelled to it by the duties of friendship.
M. Levrault, rector of the Academy of Strasburg,
one of those estimable and useful men who, while
existing, receive from their cotemporaries the tri-
bute of gratitude which they merit, requested my
co-operation in the grand literary work which he
had confided to the celebrated professors of the
Museum of Natural History of Paris ; and he suc-
ceeded in overcoming the repugnance which I have
always felt for engaging in this kind of labour.
I promised him that I would undertake for his
" Dictionnaire des Sciences Naturelles," the article
Geography of Plants. Some unforeseen occupa-
tions having prevented me from fulfilling my
promise, this article has been supplied by M. De
Candolle, with the distinguished talent that charac-
terises all his works ; and I have only added to it
researches on the numerical relations of vegetable
forms, and on the distribution of those forms in the
different climates. As a kind of compensation, I
offered to write the article Geognosy, in which
would be comprehended the description of the
several formations. The following work consists
of this article, which is now printed separately. It
is nearly of the same extent as the article formation
(terrain}, which an excellent geognost, M. de Bon-


nard, has given in the " Dictionnaire d'Histoire
Nattirelle," which is less voluminous, and is pub-
lished by M. Deterville. It appeared to me that
one could not better arrange the facts according
to their natural relations, than by devoting forty
sections to forty independent formations.

I have been particularly careful to indicate the
localities of the most interesting phenomena of po-
sition ; and I have frequently added to them the
results of my barometrical measurements. When
there has been any doubt with respect to those coun-
tries of which we have only very imperfect maps,
I have mentioned such latitudes as I had deter-
mined during my excursions in the Cordilleras.

I have explained, at the end of the work, the
principles of a geognostic pasigraphy \ and have
wished to shew that by means of a very simple
notation, and by omitting the structure and com-
position of rocks, we may express with great fa-
cility the most complicated relations that exist be-
tween the position and the periodical recurrence of
formations. This method of notation and concise
language render evident the identity of phenomena,
which, when disguised by accidental circumstances,
might at first appear to be very different. The
pasigraphic notation, which proceeds by series, and
which presents an almost algorithmic method, is
more susceptible of perfection than the imitative
or figured pasigraphy. Both these appear to me
to be important in geology ; for it is with the pa-
sigraphic language as with languages in general ;
the ideas become more clear in proportion as the
signs which express them are improved.


IN geognosy, the word formation either denotes
the manner in which a rock has been produced* or
it designates an assemblage of mineral masses so
intimately connected, that it is supposed they
were formed at the same epoch, and that they pre-
sent, in the most distant parts of the earth, the same
general relations, both of composition, and of situ-
ation with respect to each other. Thus the form-
ation of obsidian and of basalt is attributed to
subterraneous fires ; and it is also said that the
formation of transition clay-slate contains Lydian
stone, chiastolite, ampelite, and alternating beds
of black limestone, and of porphyry. The first
acceptation of the word is the most conformable
to the genius of the French language ; but it re-
lates to the origin of things, and to an uncertain
science founded on geogonic hypotheses. The
second acceptation, now generally received by
the French mineralogists, has been borrowed from
the celebrated school of WERNER, and indicates, not
what is, supposed to have been, but what now exists.


In the geognostic description of the globe, \ve
may distinguish different modes of grouping mineral
substances, as we ascend to more general ideas.
Rocks, which alternate with each other, which are
found usually together, and which display the same
relations of position, constitute the same formation ;
the union of several formations constitutes a geo-
logical series or a district (terrain) ; but the terms
rocks, formations, and terrains *, are used as
synonymous in many works on geognosy.

The diversity of rocks, and the relative position
of the beds which form the oxidated crust of the
globe, have, from the most remote times, fixed the
attention of men. Wherever the working of a mine
was directed on a mass of salt, coal, or clay iron-
stone, covered by successive beds of a different
nature, it gave rise to ideas more or less precise,
on the arrangement of the rocks peculiar to a
formation of small extent. Possessed of this local
knowledge, but influenced by prejudices having
their source in habit, miners spread themselves over
contiguous countries, and, as geognosts have often
done in our days, they decided upon the positions
of rocks, of the nature of which they were igno-
rant, according to incomplete analogies, and the
confined ideas they had originally acquired. These
errors must have had a fatal influence on the sue-

* We have no word in the English language that will accu-
rately express terrain, as used in geology by the French : it here
means a series of formations : but sometimes also terrain denotes
a tract or district consisting of a particular class of rocks,

cess of their researches. Instead of studying the
connection between two successive formations, in-
stead of extending the first types of formations
which had been impressed on their minds, they
imagined that each portion of the globe differed
in its geological constitution. This very ancient
popular opinion has been adopted and maintained
by very distinguished men in different countries ;
but when geognosy was raised to the rank of a
science, when the art of interrogating nature was
improved, and when journies to distant countries
furnished a more exact comparison between dif-
ferent formations, great and immutable laws were
recognised in the structure of the globe, and in
the superposition of rocks. The most striking
analogies in the position, composition, and the in-
cluded organic remains, of contemporary beds,
were then observed in both hemispheres ; and in
in proportion as we consider formations under a
more general point of view, their identity daily
becomes more probable.

In fact, when we examine the solid mass of our
planet, we soon perceive that some of the sub-
stances, which oryctognosy (or descriptive miner-
alogy) has made known to us separately, are found
in constant associations, and that those associations,
which are called compound rocks, do not vary, like
organised beings, according to the difference of
latitude, or of the isothermal bands under which
they are placed. Geognosts, who have travelled
through the most distant countries, have not only

B 2


found, for the most part, in the two hemispheres,
the same simple substances, quartz, feldspar, mica,
garnet, and hornblende ; but they have also observ-
ed that mountain-masses display every where the
same rocks ; that is, the same assemblages of mica,
quartz, and feldspar, in granite ; of mica, quartz, and
garnets, in mica-slate ; and of feldspar and horn-
blende, in syenite. If it has sometimes been consi-
dered, that a rock belongs exclusively to a single
portion of the globe, subsequent researches have
shown, that it also occurs in regions the most dis-
tant from its first locality. Thus we are almost
led to admit, that the formation of rocks has been
independent of the diversity of climates, and per-
haps anterior to its existence. * There is an iden-
tity even in those rocks where organized bodies
are the most variously modified.

But this identity of composition, this analogy which
is observed in the association of certain simple mi-
neral substances, may be independent of the ana-
logy of their position, and their succession. Speci-
mens of the same rocks that are found in Europe
may have been brought from the islands of the
Pacific Ocean, or the Cordilleras of the Andes ; yet
perhaps we are not authorised to conclude from
thence, that these rocks are superposed in a similar
order, and that from the discovery of one, it can be
predicted, with certainty, what the others are which
occur in the same places. Geognosts, who are

* Humboldt, Geography of Plants, 1 807, p. 1 15. Idem, Views
of the Cordilleras, vol. i. p. 122.


devoted to the study of the laws of unorganized
nature, should direct their labours towards the
recognition of these analogies of respective po-
sition. An attempt is made in the following table,
to collect what is known with most certainty of
the superposition of rocks in both continents
north and south of the equator. These types of
formations will not only be extended, but also
variously modified, in proportion to the increase
of travellers practised in geognostic observations,
and also when complete monographies of different
countries far remote from each other shall furnish
more precise results.

The developement of the order which is found
to exist in the superposition of rocks forms the
most important part of geognostic science. - It
must be allowed, that great difficulties often pre-
sent themselves in the observation of positions ;
either when we cannot arrive at the junction of two
adjoining formations, or when they do not exhibit
a regular stratification, or when their position is not
uniform, that is, when the strata of the superior
formation are not parallel to those of the lower.
But these difficulties (and it is one of the chief
advantages in observations which extend to a con-
siderable portion of our planet) diminish, or even
disappear entirely, on comparing together several
formations of great extent. The order of super-
position and the relative age of rocks are facts
susceptible of being determined, like the structure
of a plant, the proportion of the elements in a

r> 3

chemical compound, or the height of a mountain
above the level of the sea. True geognosy describes
the exterior crust of our globe such as it exists in
our days. This science has no less certainty than
the physical descriptive sciences in general : on
the contrary, whatever relates to the ancient state
of our planet, to those fluids which, it is said, held
all mineral substances in solution, to those seas
which have covered the summit of the Cordilleras,
and have afterwards disappeared, is as uncertain
as the formation of the atmosphere of the planets,
as the various migrations of plants, or the origin
of the different varieties of our species ; yet the
time is still not very remote when geolists were
occupied from choice in the solution of these pro-
blems, and with this fabulous period of the physical
history of the earth.

To render more intelligible the principle upon
which this tabular arrangement of the superposi-
tion of rocks is constructed, it ought to be preceded
by some remarks deduced from the practical study
of different formations. We shall begin by ob-
serving, that it is not easy to circumscribe the limits
of a formation. The limestone of the Jura and
the Alpine limestone, entirely distinct in one region,
appear sometimes closely connected in another.
What proves the independence of a formation, as
M. de Buch has well observed, is its immediate
superposition on rocks of a different nature, which,
consequently, ought to be considered as more
ancient. The red sandstone constitutes an inde-

pendent formation, because it is superposed indif-
ferently on black transition limestone, mica-slate,
and primitive granite ; but in a region where the
great formation of syenite and porphyry predo-
minates, those two rocks constantly alternate. It
thence results, that the syenite is subordinate to the
porphyry, and scarcely any where covers, by itself ',
the clay-slate of transition or the primitive gneiss.
But the independence of formation in no manner
excludes uniformity or concordance of position ; it
rather excludes the oryctognostic passage between
two superposed formations. Transition formations
have very often the same direction and the same
dip as primitive formations ; and yet, however near
may be the dates of their origins, we are not the
less justified in considering the anthracitous mica-
slate, or grauwacke alternating with porphyry,
as two formations independent of the granite and
the primitive gneiss which they cover. The uni-
formity of position (gleichformigkeit der lagerung)
furnishes no argument against the independence of
formations, or the considering a rock as a distinct
formation. It is because the independent form-
ations are placed indifferently on all the most
ancient rocks, (chalk upon granite, or red sandstone
on primitive mica-slate,) that the union of a great
number of observations made on very remote
points becomes eminently useful in determining
the relative age of rocks. In order to ascertain
if the zircon syenite be a transition rock, we must
shew that it is placed on formations posterior to

B 4


the black limestones containing orthoceratites. The
observations made by M. Beudant, one of the most
distinguished geologists of the present day, on the
porphyries and syenites of Hungary, may throw great
light on the formation of the American Andes : and
thus, a plant discovered in India may point out the
natural affinity between two families of plants of
equinoxial America.

The order which is followed in the tabular
arrangement of formations > is, that of the place,
and the respective position of the rocks. I do not
pretend that this situation and position has been
actually observed in every region of the earth ; I
only state them, such as they appeared to me the
most probable, after having compared a great num-
ber of facts that I have collected. I have been
guided by the idea of the relative age of rocks, in
this yet very imperfect labour which was begun
long before my voyage to the Cordilleras of the
new continent, in the year 1792, when, upon leaving
the school of Freyberg, I was appointed to the
direction of the mines in the Fichtelgebirge.

A rock may vary in composition : some of its
integrant parts may be subtracted from it, or other
substances may be found disseminated in it; and yet,
in the opinion of a geognost who has studied the
superposition of formations, the rock ought not to
change its denomination. Under the equator, as
In the north of Europe, the beds of 'a real transition
syenite lose their hornblende without becoming
another rock. The granites on the banks of the

Oronoco sometimes contain hornblende, and yet
should still be considered as primitive, although
they are not of the first or most ancient formation.
These facts have been admitted by all experienced
geognosts. The essential character of the identity
of an independent formation is its relative position,
or the place which it occupies in the general series
of formations. (Vide the classical memoir by M. de
Buch, Heber der Begriff elner gebirgsart, in the
Mag. der Naturf. 1810. p. 128 133.) For the same
reason a mere solitary fragment, an insulated speci-
men of a rock found in a collection, cannot be geo-
gnostically determined, that is, as belonging to one of
the numerous beds of which the crust of our planet
is composed. Chiastolite, the accumulation of car-
bon, or the nodules of compact limes tone in clay-slate,
nigrine or epidote in syenites (alternating with gra-
nites or porphyries), and conglomerates contained in
anthracitous mica-slate, point out transition form-
ations ; in the same manner as from the important
labours of M. Brongniart, petrifactions of shells
preserved entire indicate with precision certain
beds of the tertiary formation. But these observ-
ations, where we are guided by disseminated sub-
stances, or by characters simply zoological, com-
prehend only a small number of rocks of late origin ;
and observations of this kind often lead only to ne-
gative facts. Characters drawn from the colour of
the grain, or the small veins of carbonate of lime
that run through calcareous rocks; those that are
derived from the fissile nature, or the silky lustre


of the clay-slate ; the general aspect, and the wavy
character of the scales of mica in mica-slates ; the
size and the colour of the crystals of feldspar in
granites of various formations ; all these circum-
stances may, like every thing connected with the
habitus of minerals, lead the most acute observer
into error. No doubt black and white are the dis-
tinguishing colours of the primitive and transition
limestones; no doubt the formation of the Jura, par-
ticularly in the superior part, is generally divided
into thin beds that are whitish, with a fracture dull,
even, or nearly flat conchoidal; but, in mountains
of transition limestone, there exist insulated masses,
which in their colour and texture resemble in their
oryctognostic characters the Jura formation ; there
are also hills of the tertiary formation on the
south of the Alps, where rocks, subsequent to the
chalk, and resembling the limestone used for litho-
graphy, are found analogous to the fissile and dull
limestone of the Jura. If we prefer giving to
formations names derived from their oryctognostic
characters only, the various strata of the same com-
pound rock, when its thickness is considerable, and
when it can be traced far in the line of its direction
(streichungslinie), may appear often to belong to
different rocks, according to the points from which
specimens are taken ; consequently we can scarcely
determine any thing geognostically in collections,
but the suites of rocks, of which the mutual super-
position is known.

In advancing these opinions on the sense which


we ought to attach to the term independent form-
ations, as it relates to the following tabular arrange-
ment of their position, I am far from overlooking
the eminent services, which the most detailed
oryctognostic examination, and the profound study
of the composition of rocks, have rendered to
modern geognosy, and especially to the science of
the position, and respective situation of rocks ; al-
though, according to the important discoveries of
M. Hauy on the intimate nature of inorganic and
crystallized substances, there cannot exist, properly
speaking, a passage from one mineral substance to
another ; (Cordier, sur les Roches Volcaniques,
p. 33. ; and Berzelius, Nouveau Sy steme de Mine-
ralogie.) The passage of the base, or mass of rocks,
is not confined to those formations that are gene-
rally distinguished by the name of compound rocks.
Those considered as simple, such as transition lime-
stone, or secondary limestone, are in part amor-
phous varieties of mineral species of which there
exists a crystallized type ; and partly aggregates of
clay, carbon, &c., which cannot be accurately
determined. It is on the variable proportions of
these heterogeneous mixtures that the passages
of marly limestones to other schistose formations
are founded ; (Hauy, Tableau Comparatif de la
Cristallographie, p. 27. 30.) All the amorphous
bases of rocks, however homogeneous they may
appear at the first aspect, the bases of porphyries
and euphotides (serpentine), as well as those black
problematic masses that constitute the basanite

(basaltes) of the ancients, and which are not all
greenstone overcharged with hornblende, are ca-
pable of being submitted to a mechanical analysis.
M. Cordier has employed that analysis in the most
ingenious manner to greenstones, dolerites, and
other volcanic productions more recent. The
most minute oryctognostic examination cannot be
unimportant to the geognost who wishes to deter-
mine the relative age of formations. It is by this
kind of examination that we obtain a just idea
of the progressive manner in which, by interior
developement, (that is, by a very slow change in the
proportions of elements of the mass), the passage
takes place from one rock to another neighbouring
rock. The schists of transition, of which the struc-
ture seems, at first sight, so different from that of
porphyry or granite, present to the attentive ob-
server striking examples of insensible passages to
rocks that are granular, porphyritic, or granitoid.
At first these schists become greenish, and harder ;

Online LibraryAlexander von HumboldtA geognostical essay on the superposition of rocks, in both hemispheres [microform] → online text (page 1 of 30)