In the course of a pedestrian excursion made in the summer of 1855
I came upon the Aachensee, one of the lakes of North Tyrol, rarely
visited by tourists. It is situated about 30 miles N.E. of Innispruck,
and fills the basin of a deep valley, the upper slopes of which
are steep and richly wooded. The water of this lake is remarkably
transparent and colorless. With one exception, that of the Fountain of
Cyane—a deep pool forming the source of the little Syracusan river—it
is the most transparent body of water I remember to have seen. This
transparency revealed a very remarkable sub-aqueous landscape. The
bottom of the lake is strewn with branches and trunks of trees, which
in some parts are in almost forest-like profusion. As I was alone in
a rather solitary region, and carrying only a satchel of luggage, my
only means of further exploration were those afforded by swimming and
diving. Being an expert in these, and the July summer day very calm
and hot, I remained a long time in the water, and, by swimming very
carefully to avoid ripples, was able to survey a considerable area of
the interesting scene below.
The fact which struck me the most forcibly, and at first appeared
surprising, was the upright position of many of the large trunks,
which are of various lengths—some altogether stripped of branches,
others with only a few of the larger branches remaining. The roots of
all these are more or less buried, and they present the appearance of
having grown where they stand. Other trunks were leaning at various
angles and partly buried, some trunks and many branches lying down.
On diving I found the bottom to consist of a loamy powder of gray
color, speckled with black particles of vegetable matter—thin scaly
fragments of bark and leaves. I brought up several twigs and small
branches, and with considerable difficulty, after a succession of
immersions, succeeded in raising a branch about as thick as my arm and
about eight feet long, above three-fourths of which was buried, and
only the end above ground in the water. My object was to examine the
condition of the buried and immersed wood, and I selected this as the
oldest piece I could reach.
I found the wood very dark, the bark entirely gone, and the annual
layers curiously loosened and separable from each other, like
successive rings of bark. This continued till I had stripped the stick
to about half of its original thickness, when it became too compact to
yield to further stripping.
This structure apparently results from the easy decomposition of the
remains of the original cambium of each year, and may explain the
curious fact that so many specimens of fossilized wood exhibit the
original structure of the stem, although all the vegetable matter has
been displaced by mineral substances. If this stem had been immersed
in water capable of precipitating or depositing mineral matter in very
small interstices, the deposit would have filled up the vacant spaces
between these rings of wood as the slow decomposition of the vegetable
matter proceeded. At a later period, as the more compact wood became
decomposed, it would be substituted by a further deposit, and thus
concentric strata would be formed, presenting a mimic counterpart of
the vegetable structure.
The stick examined appeared to be a branch of oak, and was so fully
saturated with water that it sank rapidly upon being released.
On looking around the origin of this sub-aqueous forest was obvious
enough. Here and there the steep wooded slopes above the lake were
broken by long alleys or downward strips of denuded ground, where storm
torrents, or some such agency, had cleared away the trees and swept
most of them into the lake. A few uprooted trees lying at the sides of
these bare alleys told the story plainly enough. Most of these had a
considerable quantity of earth and stones adhering to their roots: this
explains the upright position of the trees in the lake.
Such trees falling into water of sufficient depth to enable them to
turn over must sink root downwards, or float in an upright position,
according to the quantity of adhering soil. The difference of depth
would tend to a more rapid penetration of water in the lower parts,
where the pressure would be greatest, and thus the upright or oblique
position of many of the floating trunks would be maintained till they
absorbed sufficient water to sink altogether.
It is generally assumed that fossil trees which are found in an upright
position have grown on the spot where they are found. The facts I have
stated show that this inference is by no means necessary, not even when
the roots are attached and some soil is found among them. In order to
account for the other surroundings of these fossil trees a very violent
hypothesis is commonly made, viz., that the soil on which they grew
sank down some hundreds of feet without disturbing them. This demands a
great strain upon the scientific imagination, even in reference to the
few cases where the trees stand perpendicular. As the majority slope
considerably the difficulty is still greater. I shall presently show
how trees like those immersed in Aachensee may have become, and are now
becoming, imbedded in rocks similar to those of the Coal Measures.
In the course of subsequent excursions on the fjords of Norway I was
reminded of the sub-aqueous forest of the Aachensee, and of the paper
which I read at the British Association meeting of 1865, of which
the above is an abstract—not by again seeing such a deposit under
water, for none of the fjords approach the singular transparency of
the lake, but by a repetition on a far larger scale of the downward
strips of denuded forest ground. Here, in Norway, their magnitude
justifies me in describing them as vegetable avalanches. They may be
seen on the Sognefjord, and especially on those terminal branches of
this great estuary, of which the steep slopes are well wooded. But
the most remarkable display that I have seen was in the course of the
magnificent, and now easily made, journey up the Storfjord and its
extension and branches, the Slyngsfjord, Sunelvsfjord, Nordalsfjord,
and Geirangerfjord. Here these avalanches of trees, with their
accompaniment of fragments of rock, are of such frequent occurrence
that sites of the farm-houses are commonly selected with reference
to possible shelter from their ravages. In spite of this they do not
always escape. In the October previous to my last visit a boat-house
and boat were swept away; and one of the most recent among the tracks
that I saw reached within twenty yards of some farm-buildings.
What has become of the millions of trees that are thus falling, and
have fallen, into the Norwegian fjords during the whole of the present
geological era? In considering this question we must remember that the
mountain slopes forming the banks of these fjords continue downwards
under the waters of the fjords which reach to depths that in some parts
are to be counted in thousands of feet.
It is evident that the loose stony and earthy matter that accompanies
the trees will speedily sink to the bottom and rest at the foot of
the slope somewhat like an ordinary sub-aerial talus, but not so the
trees. The impetus of their fall must launch them afloat and impel them
towards the middle of the estuary, where they will be spread about
and continue floating, until by saturation they become dense enough
to sink. They will thus be pretty evenly distributed over the bottom.
At the middle part of the estuary they will form an almost purely
vegetable deposit, mingled only with the very small portion of mineral
matter that is held in suspension in the apparently clear water. This
mineral matter must be distributed among the vegetable matter in the
form of impalpable particles having a chemical composition similar to
that of the rocks around. Near the shores a compound deposit must be
formed consisting of trees and fragments of leaves, twigs, and other
vegetable matter mixed with larger proportions of the mineral _débris_.
If we look a little further at what is taking place in the fjords
of Norway we shall see how this vegetable deposit will ultimately
become succeeded by an overlying mineral deposit which must ultimately
constitute a stratified rock.
All these fjords branch up into inland valleys down which pours a
brawling torrent or a river of some magnitude. These are more or less
turbid with glacier mud or other detritus, and great deposits of this
material have already accumulated in such quantity as to constitute
characteristic modern geological formations bearing the specific Norsk
name of _ören_, as _Laerdalsören_, _Sundalsören_, etc., describing
the small delta plains at the mouth of a river where it enters the
termination of the fjord, and which, from their exceptional fertility,
constitute small agricultural settlements bearing these names, which
signify the river sands of _Laerdal_, _Sundal_, etc. These deposits
stretch out into the fjord, forming extensive shallows that are
steadily growing and advancing further and further into the fjord.
One of the most remarkable examples of such deposits is that brought
by the Storelv (or Justedals Elv), which flows down the Justedal,
receiving the outpour from its glaciers, and terminates at Marifjören.
When bathing here I found an extensive sub-aqueous plain stretching
fairly across that branch of the Lyster fjord into which the Storelv
flows. The waters of the fjord are whitened to a distance of two or
three miles beyond the mouth of the river. These deposits must, if
the present conditions last long enough, finally extend to the body,
and even to the mouth, of the fjords, and thus cover the whole of the
bottom vegetable bed with a stratified rock in which will be entombed,
and well preserved, isolated specimens of the trees and other vegetable
forms corresponding to those accumulated in a thick bed below, but
which have been lying so long in the clear waters that they have become
soddened into homogeneous vegetable pulp or mud, only requiring the
pressure of solid superstratum to convert them into coal.
The specimens of trees in the upper rock, I need scarcely add, would be
derived from the same drifting as that which produced the lower pulp;
but these coming into the water at the period of its turbidity and of
the rapid deposition of mineral matter, would be sealed up one by one
as the mineral particles surrounding it subsided. Fossils of estuarine
animals would, of course, accompany these, or of fresh-water animals
where, instead of a fjord, the scene of these proceedings is an inland
lake. In reference to this I may state that at the inner extremities of
the larger Norwegian fjords the salinity of the water is so slight that
it is imperceptible to taste. I have freely quenched my thirst with the
water of the Sörfjord, the great inner branch of the Hardanger, where
pallid specimens of bladder wrack were growing on its banks.
In the foregoing matter-of-fact picture of what is proceeding on a
small scale in the Aachensee, and on a larger in Norway, we have, I
think, a natural history of the formation, not only of coal seams, but
also of the Coal Measures around and above them.
The theory which attributed our coal seams to such vegetable
accumulations as the rafts of the Mississippi is now generally
abandoned. It fails to account for the state of preservation and the
position of many of the vegetable remains associated with coal.
There is another serious objection to this theory that I have not
seen expressed. It is this: rivers bringing down to their mouths
such vegetable deltas as are supposed, would also bring considerable
quantities of earthy matter in suspension, and this would be deposited
with the trees. Instead of the 2 or 3 per cent of incombustible ash
commonly found in coal, we should thus have a quantity more nearly like
that found in bituminous shales which may thus be formed, viz., from 20
to 80 per cent.
The alternative hypothesis now more commonly accepted—that the
vegetation of our coal-fields actually grew where we find it—is also
refuted by the composition of coal-ash. If the coal consisted simply
of the vegetable matter of buried forests its composition should
correspond to that of the ashes of plants; and the refuse from our
furnaces and fireplaces would be a most valuable manure. This we know
is not the case. Ordinary coal-ash, as Bischof has shown, nearly
corresponds to that of the rocks with which it is associated; and he
says that “the conversion of vegetable substances into coal has been
effected by the agency of water;” and also that coal has been formed,
not from dwarfish mosses, sedges, and other plants which now contribute
to the growth of our peat-bogs, but from the stems and trunks of
the forest trees of the Carboniferous Period, such as _Sigillariæ_,
_Lepdodendra_, and _Coniferæ_.[14] All we know of these plants teaches
us that they could not grow in a merely vegetable soil containing but
2 or 3 per cent of mineral matter. Such must have been their soil for
hundreds of generations in order to give a depth sufficient for the
formation of the South Staffordshire ten-yard seam.
All these and other difficulties that have stood so long in the way
of a satisfactory explanation of the origin of coal appear to me to
be removed if we suppose that during the Carboniferous Period Britain
and other coal-bearing countries had a configuration similar to that
which now exists in Norway, viz., inland valleys terminating in marine
estuaries, together with inland lake basins. If to this we superadd the
warm and humid climate usually attributed to the Carboniferous Period,
on the testimony of its vegetable fossils, all the conditions requisite
for producing the characteristic deposits of the Coal Measures are
fulfilled.
We have first the under-clay due to the beginning of this state of
things, during which the hill slopes were slowly acquiring the first
germs of subsequent forest life, and were nursing them in their scanty
youth. This deposit would be a mineral mud with a few fossils and
that fragmentary or fine deposit of vegetable matter that darkens the
carboniferous shales and strips the sandstones. Such a bed of dark
consolidated mud, or fine clay, is found under every seam of coal, and
constitutes the “floor” of the coal pit. The characteristic striped
rocks—the “linstey” or “linsey” of the Welsh colliers—is just such as
I found in the course of formation in the Aachensee near the shore, as
described above.
The prevalence of estuarine and lacustrine fossils in the Coal Measures
is also in accordance with this: the constitution of coal-ash is
perfectly so. Its extreme softness and fineness of structure; its
chemical resemblance to the rocks around, and above, and below; and
oblong basin form common to our coal seams; the apparent contradiction
of such total destruction of vegetable structure common to the true
coal seams, while immediately above and below them are delicate
structures well preserved, is explained by the more rapid deposition of
the latter, and the slow soddening of the former as above described.
I do not, however, offer this as an explanation of the formation of
_every kind of coal_. On the contrary, I am satisfied that cannel coal,
and the black shales usually associated with it, have a different
origin from that of the ordinary varieties of bituminous coal. The
fact that the products of distillation of cannel and these shales form
different series of hydrocarbons from those of common coal, and that
they are nearly identical with those obtained by the distillation of
peat, is suggestive of origin in peat-bogs, or something analogous to
them.
To the above I may add the concluding sentences of the chapter on Coal
in Lyell’s “Elements of Geology.” Speaking of fossils in the Coal
Measures, he says: “The rarity of air-breathers is a very remarkable
fact when we reflect that our opportunities of examining strata _in
close connection with ancient land_ exceed in this case all that we
enjoy in regard to any other formations, whether primary, secondary, or
tertiary. We have ransacked hundreds of soils replete with the fossil
roots of trees, have dug out hundreds of erect trunks and stumps which
stood in the position in which they grew, have broken up myriads of
cubic feet of fuel still retaining its vegetable structure, and, after
all, we continue almost as much in the dark respecting the invertebrate
air-breathers of this epoch, _as if the coal had been thrown down
in mid-ocean_. The early date of the carboniferous strata cannot
explain the enigma, because we know that while the land supported a
luxuriant vegetation, the contemporaneous seas swarmed with life—with
Articulata, Mollusca, Radiata, and Fishes. We must, therefore, collect
more facts if we expect to solve a problem which, in the present state
of science, cannot but excite our wonder; and we must remember how
much the conditions of this problem have varied within the last twenty
years. We must be content to impute the scantiness of our data and our
present perplexity partly to our want of diligence as collectors, and
partly to our want of skill as interpreters. We must also confess that
our ignorance is great of the laws which govern the fossilization of
land animals, whether of low or high degree.”
The explanation of the origin of coal which I have given in the
foregoing meets all these difficulties. It shows how vast accumulations
of vegetable matter may have been formed “in close connection with
the ancient land,” and yet “as if the coal had been thrown down in
mid-ocean” as far as the remains of terrestrial animals are concerned.
It explains the nearly total absence of land shells, and of the remains
of other animals that must have lived in the forests producing the
coal, and which would have been buried there with the coal had it been
formed on land as usually supposed. It also meets the cases of the rare
and curious exceptions, seeing that occasionally a land animal would
here and there be drowned in such fjords under circumstances favorable
to its fossilization.