have crystallised on cooling, it is necessary that we should retrace
in them the same phenomena and the same laws which we still observe
at the present time. If a fluid body become solid by cooling, these
phenomena are differently modified, according to the chemical nature
of the bodies, and according to the crystalline forms which they
acquire on cooling; but the laws remain always the same. Mitscherlich
says, I am in possession of some specimens which explain several
of the phenomena so often shewn by basalt and volcanic formations.
I do not possess artificial basalt resembling the natural columnar
kind; yet the slags obtained at the furnaces of Sahla resemble basalt
so perfectly, as to deceive the most experienced eye, especially
as their cavities contain crystals of augite. But I have found at
Fahlun a bisilicate of protoxide of iron, which has in consequence
a composition analogous to that of basalt, and which has distinct
joints. In this slag we perceive that the joints, which are
parallel to the axis of the prism and to the lateral planes of the
crystals, are always perpendicular to the plane of cooling. This is
particularly observable in a specimen which was obtained by melting
the slag in a mould; on crystallizing it had several planes of
cooling, and the joints are parallel to each of these planes. The
planes of separation in basalt present exactly the same phenomenon as
this slag.
The phenomena which take place when a fluid body crystallizes may be
observed in sulphur, better than in any other body. All fluid bodies,
however, and even water, on freezing, present the same phenomena.
If a fluid body has cooled to the point at which it begins to become
solid, for example, sulphur, in a round vessel, a crust of sulphur is
not formed upon the surface of the cooled vessel, and another crust
upon the surface of the sulphur itself, as might be expected; on the
contrary, if a crystal be formed upon a point of the inner surface of
the vessel, the crystal enlarges by growing in the direction of its
axis, and the mass which surrounds the crystal remains liquid, and
sometimes cools, without the molecules arranging themselves in the
same manner as the crystal already formed. On examining the cooled
mass, we observe that it shews a lamellar texture where the crystal
was formed, and that the mass which surrounded it does not shew this
texture in the same degree. This explains how veins of large-granular
granite traverse a small-granular granite, as well as other phenomena
of the same nature.
This observation also affords an explanation of another phenomenon.
If the half of the liquid mass has become solid, and if the fluid
part be poured off, we obtain isolated crystals, which have been
formed in the fluid mass. If the fluid part be not poured off,
and be permitted to cool slowly, it contracts, as is the case
with most bodies, and the contraction produces the same effect as
the decantation; small cavities will be formed, and these will be
traversed and covered over with distinct crystals. We also observe
this phenomenon in the geodes of primitive and volcanic mountains, in
which the crystals they contain are of the same minerals as those of
which the mountains themselves are composed.
NOTE E, p. 23.
ON THE DISTRIBUTION OF BOULDER STONES IN SCOTLAND, HOLLAND, GERMANY,
SWITZERLAND, AND AMERICA.
Numerous large blocks are met with in almost every country of Europe,
and frequently far removed from their original situations. This is
frequently the case in Scotland: thus, in the Edinburgh district,
we have numerous blocks of primitive rocks, of which no fixed rocks
occur nearer than in our Highland mountains.
In the north of Holland, Germany, and the countries bordering on
the Baltic, enormous fragments of granite and syenite are scattered
within certain limits. According to Humboldt, it seems to be now
proved, that they have been carried southward, with a distribution
like that of radii from a centre, from the Scandinavian peninsula,
during some of the ancient revolutions of our globe, and that they
have not originally belonged to the granitic chains of the Hartz and
Saxony, which they approach without, however, actually attaining
their basis[374]. Born, says Humboldt, on the sandy plains of the
Baltic, and until the age of eighteen, not knowing any other rock
than these scattered blocks, I could not but feel curious to know
whether the new world presented any thing of a similar nature. I
was surprised not to find a single block of this description in the
_Llanos_ of Venezuela, although the immense plains were immediately
bordered to the south by a group of mountains entirely granitic[375],
and which presents, in its broken and almost columnar peaks, traces
of the most violent action[376]. Towards the north, the granitic
chain of the Silla of Caracas and of Portocabello is separated from
the Llanos, by a range of mountains which are schistose between
Villa de Cura and Parapara, and calcareous between the Bergantin and
Caripe. I was equally struck with the same absence of blocks upon the
banks of the Amazon. La Condamine had already affirmed, that from
the Pongo of Manseriche to the strait of Pauxis, not the smallest
stone was to be observed. Now, the basin of the Rio Nigro and of
the Amazon is also but a Llano, a plain like those of Venezuela and
Buenos Ayres, the difference consisting only in the state of the
vegetation. The two Llanos, situated at the northern and southern
extremities of South America, are covered with gramineæ; they are
Savannas destitute of trees. The intermediate Llano, that of the
Amazon, exposed to almost continual equatorial rains, is a thick
forest. I do not remember to have heard that the Pampas of Buenos
Ayres or the Savannas of the Missouri[377] and New Mexico contain
granitic blocks. The absence of this phenomenon appears general in
the new world. It is probably equally so in the Sahara in Africa;
for we must not confound rocky masses which pierce the soil in the
midst of the desert, and of which mention has often been made by
travellers, with mere scattered fragments. These facts seem to prove,
that the blocks of Scandinavian granite, which cover the sandy plains
on the southern side of the Baltic, in Westphalia, and in Holland,
are owing to a particular debacle which proceeded from the north,
to a purely local catastrophe. The old conglomerate (grès rouge),
which covers a great part of the Llanos of Venezuela and of the basin
of the Amazon, contains, without doubt, fragments of those same
primitive rocks of which the neighbouring mountains are composed;
but the convulsions of which these mountains present undoubted
evidences, do not seem to have been accompanied with circumstances
favourable to the transportation of great blocks. This geognostic
phenomenon is so much the more unexpected, that nowhere in the world
does there exist a plain so continuous, and which is prolonged with
fewer interruptions to the abrupt declivity of a purely granitic
cordillera. Before my departure from Europe, says Humboldt, I had
already been struck with the observation that there are no primitive
blocks in Lombardy, nor in the great plain of Bavaria, which is the
bottom of an ancient lake, having an elevation of 250 fathoms above
the level of the ocean. This plain is bounded on the north by the
granites of the Upper Palatinate, and on the south by the alpine
limestones, transition clay-slates, and mica-slates of the Tyrol.
Boulders, or loose blocks of alpine rocks, are found in the lower
part of the Alpine valleys, which terminate in the great principal
valley that stretches between the Alps and the Jura, from the Lake
of Geneva to the Lake Constance; and are also found almost every
where in this great principal valley. They are sometimes met with
4000 feet above the level of the sea, on the side of the Jura, facing
the Alps, and also in considerable numbers in many of the valleys of
the Jura itself. These blocks occur only on the surface, never in any
solid rock, and no one ever met with them in the subjacent strata of
sandstone, marl, or conglomerate of the hills and valleys, interposed
between the Alps and the Jura; but they are sometimes found deep in
the soil, or imbedded or surrounded with the debris formed by rivers.
The traveller is often surprised by the enormous magnitude of these
loose blocks, some of them being calculated to contain 50,000 cubic
feet. The smaller masses are distinguished from those brought down by
rivers, by their position, that is, their occurring on heights and
acclivities, where no river could ever have run. They may also be
confounded with blocks from decaying conglomerate; hence it is proper
to be on our guard, not only to distinguish these blocks from those
derived from conglomerate rocks, but also from the rolled masses
belonging to river courses.
The height at which they are found does not appear to have any
relation to their magnitude, for we often find very large blocks at
considerable heights, and also in deep valleys; and we also meet with
small masses as well in the bottoms of valleys, as high up on the
mountains.
They occur sometimes in heaps, or dispersed in single blocks; but
these relations have no connection with their magnitude, because we
often find large and small masses in the same heap, and single,
large, and small, blocks on mountain summits, and in the bottoms of
valleys. The smaller blocks are more or less rounded, but seldom
so much so as the boulders of rivers, which have been exposed to
long continued friction. The larger blocks are indeed angular, but
not sharp edged. But in examining this relation, we must carefully
distinguish whether or not the angles or edges are original, or have
been produced by subsequent, natural, or artificial causes. Very
often masses of this description are blasted with gunpowder, either
with the view of clearing the fields, or of obtaining stones for
building; and these, if left on the ground, may lead into error.
These blocks vary in their nature, some being of the primitive class,
while others belong to those of the transition and secondary classes.
In general, they appertain to rock formations, situated nearer to the
central alpine chains than those of the places where they are found.
Thus, no rocks of the transition class occur in gneiss valleys; no
alpine limestone in transition valleys; and, in general, nowhere
but in Jura, do blocks of Jura limestone make their appearance.
Therefore, all the loose blocks of rocks between the Jura and the
Alps, belong to the strata of the high chains of the Alps.
But these blocks have different characters in different districts.
The loose blocks which occur in the river basin of the Rhone, and
the Lake of Geneva, are quite different from those which lie strewed
about in the river basin of the Rhine. These, again, are equally
different from the loose blocks of the river basin of the Aare,
as those of the Aare are from the blocks of the Lake of Zurich,
and the valley of Limmat; and these in their turn are equally well
distinguished from the great accumulations in the valley of the
Reuss. It rarely happens that intermixtures take place among these
different accumulations of debris, and this is a circumstance which
must be attended to in our investigation.
It results from an accurate comparison of these loose blocks with
those mountain rocks which occur in extensive chains in the high
Alps; that the loose blocks of every known river basin agree with
the rocks which form the sides of the upper parts of those high
Alpine valleys, which are in immediate connection with these great
water basins. Thus the loose blocks of the water basin of the Rhine
are similar to the rocks of Bundten. We find in the Lake of Zurich,
and in the Limmat valley, the rocks of the Glarner land in loose
blocks. The debris in the basin of the Reuss consists of rocks of the
mountains from which the Reuss takes its rise. The loose blocks of
the water basin of the Aare are similar to the mountain rocks of the
high Alps of Bern; and the loose blocks, found in the course of the
Rhone, occur in fixed rocks in the Vallais.
It thus appears that the loose blocks are by no means irregularly
dispersed over the great valley between the Alps and the Jura, but
are distributed in the direction of distinct water basins. It also
appears, that the loose blocks are not irregularly distributed in
these different basins; on the contrary, that, in some parts of the
basin, they are accumulated in great numbers; in other places they
are rare, and in some situations none occur.
From the preceding observations, we may obtain some hints of
importance in respect of the cause of this remarkable phenomenon.
These loose blocks already occur in the alpine valleys, which open
into the great valley, between the Alps and the Jura. They are found
more abundantly in the wide parts of valleys immediately below the
narrow or contracted passes, and few occur in the narrow, steep, and
rocky parts of the valleys.
Loose blocks are found, at a greater or less height, in the smaller
lateral valleys that open into the transverse alpine valleys, which
terminate in the great valley between the Alps and the Jura. If these
lateral valleys form passes (which lead over into other valleys by
a lowering of the high mountain chain), which are not more than
4000 feet above the level of the sea, loose blocks occur, not only
in these passes, but also more or less widely distributed in the
opposite valleys. In the great principal valley which stretches
between the Alps and the Jura, from the Lake of Geneva to beyond the
Lake Constance, we find these loose blocks dispersed over all the
hills whose elevation is not more than 3000 feet above the level of
the sea; but even here the distribution of the blocks is not entirely
irregular. The largest are found on such hills and acclivities as are
opposite the mouths of the alpine valleys, in the great principal
valley. The blocks are frequently found higher on such acclivities,
than on the sides of those valleys which may be considered as a
continuation of the alpine valleys. The loose blocks are found every
where on that acclivity of the Jura range which is opposite to the
Alps, and they are found highest and largest in those places which
are directly opposite the mouths of the alpine valleys. In such
places, the blocks again attain an elevation of nearly 4000 feet
above the level of the sea; whereas, in the intermediate places,
which are most remote from the places opposite the mouths of the
alpine valleys, the blocks seldom reach at a height of 2000 feet
above the level of the sea.
In those places where the Jura chain branches into the great valley
between the Jura and the Alps, loose blocks are found in the valleys
behind the projecting chains. The Jura range is sometimes intersected
in places opposite to the Alps; and it is remarked, that loose blocks
are met with in the valleys behind these intersected portions of
the range; and that, when loose blocks occur in the Jura range, at
a distance from the Alps, it is only in such places as are directly
opposite to the intersected portions of the chain opposite to the
Alps.
The circumstance of the non-occurrence of these blocks in the
sandstone, marl, and nagelfluh, which occupy the great valley between
the Alps and the Jura, proves that that revolution of our globe, by
which these were dispersed, took place after the formation of these
rocks, and may therefore have belonged to one of the latest changes
which have contributed to the present form of the earth’s surface.
When we compare the relations of the alluvium of the rivers in
valleys with those of the loose blocks, their similarity must strike
every one. Thus, rolled masses are seldom deposited in those places
where a river forces its way through a narrow passage; but where an
expansion takes place, owing to the distance of the banks increasing,
the rolled masses are sometimes accumulated in whole banks. The same
loose blocks seldom occur in the narrow passages of the transverse
valleys in the Alps; but as soon as widenings of the valleys take
place below these narrowings, the blocks occur in abundance.
If, during a flood, a rupture takes place in the banks of a river,
where it is contracted, a part of the stream will flow out by the
lateral opening, and carry along with it rolled masses, even when the
opening in the bank does not reach to the bottom of the bed of the
river; for the mountain stream, loaded with boulders, carries them
not merely in single masses along its bottom, but the flood-water
of the stream generally attacks large sandbanks, or older beds of
rolled masses, and carries along with it, accompanied with a terrible
noise, whole masses, forces them over the lower banks, or through the
chasm in the bank, and often deposites them several feet high, on an
immediately succeeding widening of the river’s course.
In the same manner, we observe loose blocks deposited on high
situations in the lateral valleys of the great transverse valleys,
and dispersed over the passes into the neighbouring valleys. The
height of the lateral deposites of loose blocks, and their position
in the passes, and their passing into neighbouring valleys, are facts
which assist us in judging of the extent of the power that may have
acted during their transportation.
The striking agreement observable in the phenomena of the
distribution of the loose blocks from the interior Alpine valleys to
the interior valleys of the Jura, with those in the rolled masses
carried along by rivers, must lead every one, who reflects on this
interesting phenomenon, to the hypothesis, that these blocks may have
been deposited in their present situations by an overwhelming flood,
which burst from the Alps. It is true that this opinion is liable to
many objections; but still it contains a more plausible explanation
of the phenomenon than any other with which we are acquainted.
The loose blocks, in the different river-districts, being in general
separated from each other, or if any intermixture takes place of the
rolled masses of one valley with that of another, it being only on
their edges, it is highly probable that the floods which burst from
these valleys, and carried along with them the masses of rocks, may
have been simultaneous, by which the flow of the one basin would
bound and limit that of the other, and thus prevent the water-flood
of one basin flowing into the neighbouring ones.
The contemporaneous occurrence of these different floods from the
Alpine valleys, can alone, on this hypothesis, explain why this
aqueous flood was so generally and so highly accumulated in the great
valleys between the Alps and the Jura, as to reach the height of most
of the sandstone mountains, and to a great elevation in the Jura,
where many blocks are found deposited. But if the contemporaneous
occurrence of these floods is proved by the facts already enumerated,
to what cause are we to refer this simultaneous bursting of floods of
water from so many Alpine valleys?
We observe, on the north-western side of the chain of the Alps,
numerous openings, which, by their structure, seem to point out the
action of violent floods. Let us suppose the numerous valleys, in the
districts already described, closed at their present entrances, or
openings, as would seem from their structure to have been formerly
the case; the consequence of this arrangement would be the filling
of the Alpine valleys with water, to the height of the lowest passes
among the mountains, and thus an enormous accumulation of water
would take place. This great body of water, if let loose at once, by
the bursting of the lower extremities of the valleys, would form a
flood which would sweep across the sandstone mountains, between the
Alps and the Jura range, and even ascend high on the Jura itself.
This flood of water, moving, probably, at the rate of 200 feet in a
second, and loaded with debris of rocks, would carry masses, even
these having a magnitude of 50,000 cubical feet, some thousand feet
high, on the Jura range[378]. Geologists maintain, that the blocks
or boulders met with in other countries, and arranged as those in
Switzerland, have been deposited where we now find them, by the
bursting of lakes; while those found on the shores of the Baltic, are
conjectured to have been transported by a great rush of water caused
by the sudden elevation of the land of Scandinavia. Another opinion
has its advocates, which maintains that these boulders have been
spread over different countries by the waters of the _deluge_.
NOTE F, p. 26.
ON THE ALLUVIAL LAND OF THE DANISH ISLANDS IN THE BALTIC, AND ON THE
COAST OF SLESWIGH.
In this section, Cuvier gives a clear and distinct account of several
kinds of alluvial formations. M. De Luc, in the first volume of his
Geological Travels, describes the alluvial formations that cover
and bound many of the islands in the Baltic, and upon the coast of
Denmark, and gives so interesting an account of the modes followed by
the inhabitants, in preserving these alluvial deposites, that we feel
pleasure in communicating it to our readers.
“During my stay at Husum, I had the advantage of passing my
evenings very agreeably and profitably at the house of M.
Hartz, with his own family, and two Danish officers, Major
Behmann, commandant at Husum, and Captain Baron de Barackow. The
conversation often turned on the objects of my excursions, and
particularly on the natural history of the _coasts_ and of the
_islands_; respecting which, M. Hartz obligingly undertook to
give me extracts from the chronicles of the country. This led
us to speak of the Danish islands; and those officers giving me
such descriptions of them, as were very interesting to my object,
I begged their permission to write down, in their presence, the
principal circumstances which they communicated to me. These will
form the first addition to my own observations; I shall afterwards
proceed to the information which I obtained from M. Hartz.
The two principal islands of the Danish Archipelago, those of
Funen and Seeland (or Zeeland), as well as some small islands
in the Kattegate, namely, Lenoe, Anholt, and Samsoe, are hilly,
and principally composed of _geest_[379]; and in these are found
_gravel_ and _blocks of granite_, and of other stones of that
class, exactly in the same manner as in the country which I have
lately described, and its _islands_ in the North Sea. On the
borders of the two first of these Danish islands, there are also
_blocks_ in the sea; but only in front of _abrupt_ coasts, as is
the case with the islands of Poel and Rugen, and along the coasts
of the Baltic. The lands added to these islands of _geest_ are in
most part composed of the _sand_ of the sea, the land-waters there
being very inconsiderable; and to the south of them have been
formed several islands of the same nature, the chief of which are
Laland and Falster, near Seeland. These, like the marsh islands
in the North Sea, are sand-banks accumulated by the waves, and,
when covered with grass, continuing to be farther raised by the
sediments deposited between its blades. In the Baltic, where there
are no sensible tides, such islands may be inhabited without dikes,
as well as the extensions of the coasts; because, being raised to
the highest level of that sea, while their declivity under water
is very small, and being also more firm in their composition, the
waves die away on their shores; and if, in any extraordinary case,
the sea rises over them, it leaves on them fresh deposits, which
increase their heights. These soils are all perfectly _horizontal_,
like those added to the coasts of the Continent.
Some of these islands approach entirely, or in part, to the nature
of that of Rugen. This island of Seeland, on that side which is
called Hedding, has a promontory composed of strata of _chalk_
with its flints. The island of Moen (or Mona), on the south of
the latter, has a similar promontory near Maglebye and Mandemark;
and the island of Bornholm, the easternmost of those belonging
to Denmark, contains strata of _coal_, covered by others of
_sandstone_. Phenomena like these, evident symptoms of the most
violent catastrophes at the bottom of the ancient sea, proceeding,
as I think I have clearly shewn, from the subsidence and angular
motions of large masses of strata, which must have forced out the
interior fluids with the utmost impetuosity, it is not surprising
that so many fragments of the lowermost strata are found dispersed
over this great theatre of ruins.