ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.
Volcanic rocks of the Older Pliocene period--Tuscany--Rome--Volcanic
region of Olot in Catalonia--Cones and lava-currents--Ravines and
ancient gravel-beds--Jets of air called Bufadors--Age of the
Catalonian volcanos--Miocene period--Brown-coal of the Eifel and
contemporaneous trachytic breccias--Age of the brown-coal--Peculiar
characters of the volcanos of the upper and lower Eifel--Lake
craters--Trass--Hungarian volcanos.
_Older Pliocene period--Tuscany._--In Tuscany, as at Radicofani, Viterbo,
and Aquapendente, and in the Campagna di Roma, submarine volcanic tuffs are
interstratified with the Older Pliocene strata of the Subapennine hills, in
such a manner as to leave no doubt that they were the products of eruptions
which occurred when the shelly marls and sands of the Subapennine hills
were in the course of deposition.
_Catalonia._--Geologists are far from being able, as yet, to assign to
each of the volcanic groups scattered over Europe a precise
chronological place in the tertiary series; but I shall describe here,
as probably referable to some part of the Pliocene period, a district of
extinct volcanos near Olot, in the north of Spain, which is little
known, and which I visited in the summer of 1830.
The whole extent of country occupied by volcanic products in Catalonia is
not more than fifteen geographical miles from north to south, and about six
from east to west. The vents of eruption range entirely within a narrow
band running north and south; and the branches, which are represented as
extending eastward in the map, are formed simply of two lava-streams--those
of Castell Follit and Cellent.
[Illustration: Fig. 470. Volcanic district of Catalonia.]
Dr. Maclure, the American geologist, was the first who made known the
existence of these volcanos[409-A]; and, according to his description, the
volcanic region extended over twenty square leagues, from Amer to Massanet.
I searched in vain in the environs of Massanet, in the Pyrenees, for traces
of a lava-current; and I can say, with confidence, that the adjoining map
gives a correct view of the true area of the volcanic action.
_Geological structure of the district._--The eruptions have burst entirely
through fossiliferous rocks, composed in great part of grey and greenish
sandstone and conglomerate, with some thick beds of nummulitic limestone.
The conglomerate contains pebbles of quartz, limestone, and Lydian stone.
This system of rocks is very extensively spread throughout Catalonia; one
of its members being a red sandstone, to which the celebrated salt-rock of
Cardona, usually considered as of the cretaceous era, is subordinate.
Near Amer, in the Valley of the Ter, on the southern borders of the region
delineated in the map, primary rocks are seen, consisting of gneiss,
mica-schist, and clay-slate. They run in a line nearly parallel to the
Pyrenees, and throw off the fossiliferous strata from their flanks, causing
them to dip to the north and north-west. This dip, which is towards the
Pyrenees, is connected with a distinct axis of elevation, and prevails
through the whole area described in the map, the inclination of the beds
being sometimes at an angle of between 40 and 50 degrees.
It is evident that the physical geography of the country has undergone
no material change since the commencement of the era of the volcanic
eruptions, except such as has resulted from the introduction of new
hills of scoriae, and currents of lava upon the surface. If the lavas
could be remelted and poured out again from their respective craters,
they would descend the same valleys in which they are now seen, and
re-occupy the spaces which they at present fill. The only difference in
the external configuration of the fresh lavas would consist in this,
that they would nowhere be intersected by ravines, or exhibit marks of
erosion by running water.
_Volcanic cones and lavas._--There are about fourteen distinct cones with
craters in this part of Spain, besides several points whence lavas may have
issued; all of them arranged along a narrow line running north and south,
as will be seen in the map. The greatest number of perfect cones are in the
immediate neighbourhood of Olot, some of which (Nos. 2, 3. and 5.) are
represented in the annexed woodcut; and the level plain on which that town
stands has clearly been produced by the flowing down of many lava-streams
from those hills into the bottom of a valley, probably once of considerable
depth, like those of the surrounding country.
[Illustration: Fig. 471. View of the Volcanos around Olot in Catalonia.]
In this drawing an attempt is made to represent, by the shading of the
landscape, the different geological formations of which the country is
composed.[410-A] The white line of mountains (No. 1.) in the distance is
the Pyrenees, which are to the north of the spectator, and consist of
hypogene and ancient fossiliferous rocks. In front of these are the
fossiliferous formations (No. 4.) which are in shade. The hills 2, 3. 5.
are volcanic cones, and the rest of the ground on which the sunshine falls
is strewed over with volcanic ashes and lava.
The Fluvia, which flows near the town of Olot, has cut to the depth of only
40 feet through the lavas of the plain before mentioned. The bed of the
river is hard basalt; and at the bridge of Santa Madalena are seen two
distinct lava-currents, one above the other, separated by a horizontal bed
of scoriae 8 feet thick.
In one place, to the south of Olot, the even surface of the plain is
broken by a mound of lava, called the "Bosque de Tosca," the upper part
of which is scoriaceous, and covered with enormous heaps of fragments of
basalt, more or less porous. Between the numerous hummocks thus formed
are deep cavities, having the appearance of small craters. The whole
precisely resembles some of the modern currents of Etna, or that of
Come, near Clermont; the last of which, like the Bosque de Tosca,
supports only a scanty vegetation.
Most of the Catalonian volcanos are as entire as those in the neighbourhood
of Naples, or on the flanks of Etna. One of these, called Montsacopa (No.
3. fig. 471.), is of a very regular form, and has a circular depression or
crater at the summit. It is chiefly made up of red scoriae,
undistinguishable from that of the minor cones of Etna. The neighbouring
hills of Olivet (No. 2.) and Garrinada (No. 5.) are of similar composition
and shape. The largest crater of the whole district occurs farther to the
east of Olot, and is called Santa Margarita. It is 455 feet deep, and about
a mile in circumference. Like Astroni, near Naples, it is richly covered
with wood, wherein game of various kinds abounds.
[Illustration: Fig. 472. Cross section.
_a._ Secondary conglomerate.
_b._ Thin seams of volcanic sand and scoriae.]
Although the volcanos of Catalonia have broken out through sandstone,
shale, and limestone, as have those of the Eifel, in Germany, to be
described in the sequel, there is a remarkable difference in the nature of
the ejections composing the cones in these two regions. In the Eifel, the
quantity of pieces of sandstone and shale thrown out from the vents is
often so immense as far to exceed in volume the scoriae, pumice, and lava;
but I sought in vain in the cones near Olot for a single fragment of any
extraneous rock; and Don Francisco Bolos, an eminent botanist of Olot,
informed me that he had never been able to detect any. Volcanic sand and
ashes are not confined to the cones, but have been sometimes scattered by
the wind over the country, and drifted into narrow valleys, as is seen
between Olot and Cellent, where the annexed section (fig. 472.) is exposed.
The light cindery volcanic matter rests in thin regular layers, just as it
alighted on the slope formed by the solid conglomerate. No flood could have
passed through the valley since the scoriae fell, or these would have been
for the most part removed.
[Illustration: Fig. 473. Section above the bridge of Cellent.
_a._ Scoriaceous lava.
_b._ Schistose basalt.
_c._ Columnar basalt.
_d._ Scoria, vegetable soil, and alluvium.
_e._ Nummulitic limestone.
_.f_ Micaceous grey sandstone.]
The currents of lava in Catalonia, like those of Auvergne, the Vivarais,
Iceland, and all mountainous countries, are of considerable depth in narrow
defiles, but spread out into comparatively thin sheets in places where the
valleys widen. If a river has flowed on nearly level ground, as in the
great plain near Olot, the water has only excavated a channel of slight
depth; but where the declivity is great, the stream has cut a deep section,
sometimes by penetrating directly through the central part of a
lava-current, but more frequently by passing between the lava and the
secondary rock which bounds the valley. Thus, in the accompanying section,
at the bridge of Cellent, six miles east of Olot, we see the lava on one
side of the small stream; while the inclined stratified rocks constitute
the channel and opposite bank. The upper part of the lava at that place, as
is usual in the currents of Etna and Vesuvius, is scoriaceous; farther down
it becomes less porous, and assumes a spheroidal structure; still lower it
divides in horizontal plates, each about 2 inches in thickness, and is more
compact. Lastly, at the bottom is a mass of prismatic basalt about 5 feet
thick. The vertical columns often rest immediately on the subjacent
secondary rocks; but there is sometimes an intervention of such sand and
scoriae as cover the country during volcanic eruptions, and which when
unprotected, as here, by superincumbent lava, is washed away from the
surface of the land. Sometimes, the bed _d_ contains a few pebbles and
angular fragments of rock; in other places fine earth, which may have
constituted an ancient vegetable soil.
In several localities, beds of sand and ashes are interposed between the
lava and subjacent stratified rock, as may be seen if we follow the course
of the lava-current which descends from Las Planas towards Amer, and stops
two miles short of that town. The river there has often cut through the
lava, and through 18 feet of underlying limestone. Occasionally an
alluvium, several feet thick, is interspersed between the igneous and
marine formation; and it is interesting to remark that in this, as in other
beds of pebbles occupying a similar position, there are no rounded
fragments of lava; whereas in the most modern gravel-beds of rivers of this
country, volcanic pebbles are abundant.
The deepest excavation made by a river through lava, which I observed in
this part of Spain, is that seen in the bottom of a valley near San Feliu
de Pallerols, opposite the Castell de Stolles. The lava there has filled up
the bottom of a valley, and a narrow ravine has been cut through it to the
depth of 100 feet. In the lower part the lava has a columnar structure. A
great number of ages were probably required for the erosion of so deep a
ravine; but we have no reason to infer that this current is of higher
antiquity than those of the plain near Olot. The fall of the ground, and
consequent velocity of the stream, being in this case greater, a more
considerable volume of rock may have been removed in the same time.
[Illustration: Fig. 474. Section at Castell Follit.
A. Church and town of Castell Follit, overlooking precipices of basalt.
B. Small island, on each side of which branches of the river Teronel flow
to meet the Fluvia.
_c._ Precipice of basaltic lava, chiefly columnar, about 130 feet
in height.
_d._ Ancient alluvium, underlying the lava-current.
_e._ Inclined strata of secondary sandstone.]
I shall describe one more section to elucidate the phenomena of this
district. A lava-stream, flowing from a ridge of hills on the east of
Olot, descends a considerable slope, until it reaches the valley of the
river Fluvia. Here, for the first time, it comes in contact with running
water, which has removed a portion, and laid open its internal structure
in a precipice about 130 feet in height, at the edge of which stands the
town of Castell Follit.
By the junction of the rivers Fluvia and Teronel, the mass of lava has been
cut away on two sides; and the insular rock B (fig. 474.) has been left,
which was probably never so high as the cliff A, as it may have constituted
the lower part of the sloping side of the original current.
From an examination of the vertical cliffs, it appears that the upper part
of the lava on which the town is built is scoriaceous, passing downwards
into a spheroidal basalt; some of the huge spheroids being no less than 6
feet in diameter. Below this is a more compact basalt, with crystals of
olivine. There are in all five distinct ranges of basalt, the uppermost
spheroidal, and the rest prismatic, separated by thinner beds not columnar,
and some of which are schistose. These were probably formed by successive
flows of lava, whether during the same eruption or at different periods.
The whole mass rests on alluvium, ten or twelve feet in thickness, composed
of pebbles of limestone and quartz, but without any intermixture of igneous
rocks; in which circumstance alone it appears to differ from the modern
gravel of the Fluvia.
_Bufadors._--The volcanic rocks near Olot have often a cavernous structure,
like some of the lavas of Etna; and in many parts of the hill of Batet, in
the environs of the town, the sound returned by the earth, when struck, is
like that of an archway. At the base of the same hill are the mouths of
several subterranean caverns, about twelve in number, which are called in
the country "bufadors," from which a current of cold air issues during
summer, but which in winter is said to be scarcely perceptible. I visited
one of these bufadors in the beginning of August, 1830, when the heat of
the season was unusually intense, and found a cold wind blowing from it,
which may easily be explained; for as the external air, when rarefied by
heat, ascends, the pressure of the colder and heavier air of the caverns in
the interior of the mountain causes it to rush out to supply its place.
In regard to the age of these Spanish volcanos, attempts have been made
to prove, that in this country, as well as in Auvergne and the Eifel,
the earliest inhabitants were eye-witnesses to the volcanic action. In
the year 1421, it is said, when Olot was destroyed by an earthquake, an
eruption broke out near Amer, and consumed the town. The researches of
Don Francisco Bolos have, I think, shown, in the most satisfactory
manner, that there is no good historical foundation for the latter part
of this story; and any geologist who has visited Amer must be convinced
that there never was any eruption on that spot. It is true that, in the
year above mentioned, the whole of Olot, with the exception of a single
house, was cast down by an earthquake; one of those shocks which, at
distant intervals during the last five centuries, have shaken the
Pyrenees, and particularly the country between Perpignan and Olot, where
the movements, at the period alluded to, were most violent.
The annihilation of the town may, perhaps, have been due to the cavernous
nature of the subjacent rocks; for Catalonia is beyond the line of those
European earthquakes which have, within the period of history, destroyed
towns throughout extensive areas.
As we have no historical records, then, to guide us in regard to the
extinct volcanos, we must appeal to geological monuments. The annexed
diagram will present to the reader, in a synoptical form, the results
obtained from numerous sections.
The more modern alluvium (_d_) is partial, and has been formed by the
action of rivers and floods upon the lava; whereas the older gravel (_b_)
was strewed over the country before the volcanic eruptions. In neither have
any organic remains been discovered; so that we can merely affirm, as yet,
that the volcanos broke out after the elevation of some of the newest rocks
of the nummulitic (Eocene?) series of Catalonia, and before the formation
of an alluvium (_d_) of unknown date. The integrity of the cones merely
shows that the country has not been agitated by violent earthquakes, or
subjected to the action of any great transient flood since their origin.
[Illustration: Fig. 475. Superposition of rocks in the volcanic
district of Catalonia.
_a._ Sandstone and nummulitic limestone.
_b._ Older alluvium without volcanic pebbles.
_c._ Cones of scoriae and lava.
_d._ Newer alluvium.]
East of Olot, on the Catalonian coast, marine tertiary strata occur, which,
near Barcelona, attain the height of about 500 feet. From the shells which
I collected, these strata appear to correspond in age with the Subapennine
beds; and it is not improbable that their upheaval from beneath the sea
took place during the period of volcanic eruption round Olot. In that case
these eruptions may have occurred at the close of the Older Pliocene era,
but perhaps subsequently, for their age is at present quite uncertain.
_Miocene period--Volcanic rocks of the Eifel._--The chronological relations
of the volcanic rocks of the Lower Rhine and the Eifel are also involved in
a considerable degree of ambiguity; but we know that some portion of them
were coeval with the deposition of a tertiary formation, called
"Brown-Coal" by the Germans, which probably belongs to the Miocene, if not
referable to the Upper Eocene, epoch.
This Brown-Coal is seen on both sides of the Rhine, in the neighbourhood
of Bonn, resting unconformably on highly inclined and vertical strata of
Silurian and Devonian rocks. Its position, and the space occupied by the
volcanic rocks, both of the Westerwald and Eifel, will be seen by
referring to the map in the next page (fig. 476.), for which I am
indebted to Mr. Horner, whose residence in the country has enabled him
to verify the maps of MM. Noeggerath and Von Oeynhausen, from which that
now given has been principally compiled.
The Brown-Coal formation consists of beds of loose sand, sandstone, and
conglomerate, clay with nodules of clay-ironstone, and occasionally silex.
Layers of light brown, and sometimes black lignite, are interstratified
with the clays and sands, and often irregularly diffused through them. They
contain numerous impressions of leaves and stems of trees, and are
extensively worked for fuel, whence the name of the formation.
[Illustration: Fig. 476. Map of the volcanic region of the Upper
and Lower Eifel.
____1____2____3____4____5 English Miles.
Volcanic District {A. of the Upper Eifel.
{B. of the Lower Eifel.
Trachyte.
Points of eruption, with craters and scoriae.
Basalt.
Brown-coal.
_N.B._ The country in that part of the map which is left blank is composed
of inclined Silurian and Devonian rocks.]
In several places, layers of trachytic tuff are interstratified, and in
these tuffs are leaves of plants identical with those found in the
brown-coal, showing that, during the period of the accumulation of the
latter, some volcanic products were ejected.
The varieties of wood in the lignite are said to belong entirely to
dicotyledonous trees; but among the impressions of leaves, collected by Mr.
Horner, some were referred by Mr. Lindley to a palm, perhaps of the genus
_Chamaerops_, and others resembled the _Cinnamomum dulce_, and _Podocarpus
macrophylla_, which would also indicate a warm climate.[416-A]
The other organic remains of the brown-coal are principally fishes; they
are found in a bituminous shale, called paper-coal, from being divisible
into extremely thin leaves. The individuals are very numerous; but they
appear to belong to about five species, which M. Agassiz informs me are all
extinct, and hitherto peculiar to this brown-coal. They belong to the
freshwater genera _Leuciscus_, _Aspius_, and _Perca_. The remains of frogs
also, of an extinct species, have been discovered in the paper-coal; and a
complete series may be seen in the museum at Bonn, from the most imperfect
state of the tadpole to that of the full-grown animal. With these a
salamander, scarcely distinguishable from the recent species, has been
found, and several remains of insects.
The brown-coal was evidently a freshwater formation; but fossil shells have
been scarcely ever found in it; although near Marienforst, in the vicinity
of Bonn, large blocks have been met with of a white opaque chert,
containing numerous casts of freshwater shells, which appear to belong to
_Planorbis rotundatus_ and _Limnea longiscata_, two species common both to
the Middle and Upper Eocene periods. It is very probable that the
brown-coal may be connected in age with those fluvio-marine formations
which are found in higher parts of the valley of the Rhine, as at Mayence
before mentioned (p. 177.).
A vast deposit of gravel, chiefly composed of pebbles of white quartz, but
containing also a few fragments of other rocks, lies over the brown-coal
formation, forming sometimes only a thin covering, at others attaining a
thickness of more than 100 feet. This gravel is very distinct in character
from that now forming the bed of the Rhine. It is called "Kiesel gerolle"
by the Germans, often reaches great elevations, and is covered in several
places with volcanic ejections. It is evident that the country has
undergone great changes in its physical geography since this gravel was
formed; for its position has scarcely any relation to the existing drainage
of the country, and all the more modern volcanic rocks of the same region
are posterior to it in date.
Some of the newest beds of volcanic sand, pumice, and scoriae are
interstratified near Andernach and elsewhere with the loam called loess,
which was before described as being full of land and freshwater shells of
recent species, and referable to the Post-Pliocene period. I have before
hinted (see p. 118.) that this intercalation of volcanic matter between
beds of loess may possibly be explained without supposing the last
eruptions of the Lower Eifel to have taken place so recently as the era of
the deposition of the loess; but farther researches should be directed to
the investigation of this curious point.
The igneous rocks of the Westerwald, and of the mountains called the
Siebengebirge, consist partly of basaltic and partly of trachytic lavas,
the latter being in general the more ancient of the two. There are many
varieties of trachyte, some of which are highly crystalline, resembling a
coarse-grained granite, with large separate crystals of felspar. Trachytic
tuff is also very abundant. These formations, some of which were certainly
contemporaneous with the origin of the brown-coal, were the first of a long
series of eruptions, the more recent of which happened when the country
had acquired nearly all its present geographical features.
_Newer volcanos of the Eifel.--Lake-craters._--As I recognized in the
more modern volcanos of the Eifel characters distinct from any
previously observed by me in those of France, Italy, or Spain, I shall
briefly describe them. The fundamental rocks of the district are grey
and red sandstones and shales, with some associated limestones, replete
with fossils of the Devonian or Old Red Sandstone group. The volcanos
broke out in the midst of these inclined strata, and when the present
systems of hills and valleys had already been formed. The eruptions
occurred sometimes at the bottom of deep valleys, sometimes on the
summit of hills, and frequently on intervening platforms. In travelling
through this district we often fall upon them most unexpectedly, and may
find ourselves on the very edge of a crater before we had been led to
suspect that we were approaching the site of any igneous outburst. Thus,
for example, on arriving at the village of Gemund, immediately south of
Daun, we leave the stream, which flows at the bottom of a deep valley in
which strata of sandstone and shale crop out. We then climb a steep
hill, on the surface of which we see the edges of the same strata
dipping inwards towards the mountain. When we have ascended to a
considerable height, we see fragments of scoriae sparingly scattered over
the surface; till, at length, on reaching the summit, we find ourselves
suddenly on the edge of a _tarn_, or deep circular lake-basin.
[Illustration: Fig. 477. The Gemunder Maar.]
[Illustration: Fig. 478. Cross section.
_a._ Village of Gemund.
_b._ Gemunder Maar.
_c._ Weinfelder Maar.
_d._ Schalkenmehren Maar.]
This, which is called the Gemunder Maar, is the first of three lakes which
are in immediate contact, the same ridge forming the barrier of two
neighbouring cavities (see fig. 477.). On viewing the first of these, we
recognize the ordinary form of a crater, for which we have been prepared
by the occurrence of scoriae scattered over the surface of the soil. But on
examining the walls of the crater we find precipices of sandstone and shale
which exhibit no signs of the action of heat; and we look in vain for those
beds of lava and scoriae, dipping in opposite directions on every side,
which we have been accustomed to consider as characteristic of volcanic
craters. As we proceed, however, to the opposite side of the lake, and
afterwards visit the craters _c_ and _d_ (fig. 478.), we find a
considerable quantity of scoriae and some lava, and see the whole surface of
the soil sparkling with volcanic sand, and strewed with ejected fragments
of half-fused shale, which preserves its laminated texture in the interior,
while it has a vitrified or scoriform coating.
A few miles to the south of the lakes above mentioned occurs the Pulvermaar
of Gillenfeld, an oval lake of very regular form, and surrounded by an
unbroken ridge of fragmentary materials, consisting of ejected shale and
sandstone, and preserving a uniform height of about 150 feet above the
water. The side slope in the interior is at an angle of about 45 degrees;
on the exterior, of 35 degrees. Volcanic substances are intermixed very
sparingly with the ejections, which in this place entirely conceal from
view the stratified rocks of the country.[419-A]
[Illustration: Fig. 479. Outline of Mosenberg, Upper Eifel.]
The Meerfelder Maar is a cavity of far greater size and depth, hollowed
out of similar strata; the sides presenting some abrupt sections of
inclined secondary rocks, which in other places are buried under vast
heaps of pulverized shale. I could discover no scoriae amongst the
ejected materials, but balls of olivine and other volcanic substances
are mentioned as having been found.[419-B] This cavity, which we must
suppose to have discharged an immense volume of gas, is nearly a mile in
diameter, and is said to be more than one hundred fathoms deep. In the
neighbourhood is a mountain called the Mosenberg, which consists of red
sandstone and shale in its lower parts, but supports on its summit a
triple volcanic cone, while a distinct current of lava is seen
descending the flanks of the mountain. The edge of the crater of the
largest cone reminded me much of the form and characters of that of
Vesuvius; but I was much struck with the precipitous and almost
overhanging wall or parapet which the scoriae presented towards the
exterior, as at _a b_ (fig. 479.); which I can only explain by supposing
that fragments of red-hot lava, as they fell round the vent, were
cemented together into one compact mass, in consequence of continuing to
be in a half-melted state.
If we pass from the Upper to the Lower Eifel, from A to B (see map, p.
416.), we find the celebrated lake-crater of Laach, which has a greater
resemblance than any of those before mentioned to the Lago di Bolsena, and
others in Italy--being surrounded by a ridge of gently sloping hills,
composed of loose tuffs, scoriae, and blocks of a variety of lavas.
One of the most interesting volcanos on the left bank of the Rhine is
called the Roderberg. It forms a circular crater nearly a quarter of a mile
in diameter, and 100 feet deep, now covered with fields of corn. The highly
inclined strata of ancient sandstone and shale rise even to the rim of one
side of the crater; but they are overspread by quartzose gravel, and this
again is covered by volcanic scoriae and tufaceous sand. The opposite wall
of the crater is composed of cinders and scorified rock, like that at the
summit of Vesuvius. It is quite evident that the eruption in this case
burst through the sandstone and alluvium which immediately overlies it; and
I observed some of the quartz pebbles mixed with scoriae on the flanks of
the mountain, as if they had been cast up into the air, and had fallen
again with the volcanic ashes. I have already observed, that a large part
of this crater has been filled up with loess (p. 118.).
The most striking peculiarity of a great many of the craters above
described, is the absence of any signs of alteration or torrefaction in
their walls, when these are composed of regular strata of ancient sandstone
and shale. It is evident that the summits of hills formed of the
above-mentioned stratified rocks have, in some cases, been carried away by
gaseous explosions, while at the same time no lava, and often a very small
quantity only of scoriae, has escaped from the newly formed cavity. There
is, indeed, no feature in the Eifel volcanos more worthy of note, than the
proofs they afford of very copious aeriform discharges, unaccompanied by
the pouring out of melted matter, except, here and there, in very
insignificant volume. I know of no other extinct volcanos where gaseous
explosions of such magnitude have been attended by the emission of so small
a quantity of lava. Yet I looked in vain in the Eifel for any appearances
which could lend support to the hypothesis, that the sudden rushing out of
such enormous volumes of gas had ever lifted up the stratified rocks
immediately around the vent, so as to form conical masses, having their
strata dipping outwards on all sides from a central axis, as is assumed in
the theory of elevation craters, alluded to at the end of Chap. XXIX.
_Trass._--In the Lower Eifel, eruptions of trachytic lava preceded the
emission of currents of basalt, and immense quantities of pumice were
thrown out wherever trachyte issued. The tufaceous alluvium called
_trass_, which has covered large areas in this region and choked up some
valleys now partially re-excavated, is unstratified. Its base consists
almost entirely of pumice, in which are included fragments of basalt and
other lavas, pieces of burnt shale, slate, and sandstone, and numerous
trunks and branches of trees. If this trass was formed during the period
of volcanic eruptions it may perhaps have originated in the manner of
the moya of the Andes.
We may easily conceive that a similar mass might now be produced, if a
copious evolution of gases should occur in one of the lake basins. The
water might remain for weeks in a state of violent ebullition, until it
became of the consistency of mud, just as the sea continued to be charged
with red mud round Graham's Island, in the Mediterranean, in the year 1831.
If a breach should then be made in the side of the cone, the flood would
sweep away great heaps of ejected fragments of shale and sandstone, which
would be borne down into the adjoining valleys. Forests might be torn up by
such a flood, and thus the occurrence of the numerous trunks of trees
dispersed irregularly through the trass, can be explained.
_Hungary._--M. Beudant, in his elaborate work on Hungary, describes five
distinct groups of volcanic rocks, which although nowhere of great
extent, form striking features in the physical geography of that
country, rising as they do abruptly from extensive plains composed of
tertiary strata. They may have constituted islands in the ancient sea,
as Santorin and Milo now do in the Grecian Archipelago; and M. Beudant
has remarked that the mineral products of the last-mentioned islands
resemble remarkably those of the Hungarian extinct volcanos, where many
of the same minerals as opal, calcedony, resinous silex (_silex
resinite_), pearlite, obsidian, and pitchstone abound.
The Hungarian lavas are chiefly felspathic, consisting of different
varieties of trachyte; many are cellular, and used as millstones; some so
porous and even scoriform as to resemble those which have issued in the
open air. Pumice occurs in great quantity; and there are conglomerates, or
rather breccias, wherein fragments of trachyte are bound together by
pumiceous tuff, or sometimes by silex.
It is probable that these rocks were permeated by the waters of hot
springs, impregnated, like the Geysers, with silica; or in some instances,
perhaps, by aqueous vapours, which, like those of Lancerote, may have
precipitated hydrate of silica.
By the influence of such springs or vapours the trunks and branches of
trees washed down during floods, and buried in tuffs on the flanks of the
mountains, are supposed to have become silicified. It is scarcely possible,
says M. Beudant, to dig into any of the pumiceous deposits of these
mountains without meeting with opalized wood, and sometimes entire
silicified trunks of trees of great size and weight.
It appears from the species of shells collected principally by M. Boue,
and examined by M. Deshayes, that the fossil remains imbedded in the
volcanic tuffs, and in strata alternating with them in Hungary, are of
the Miocene type, and not identical, as was formerly supposed, with the
fossils of the Paris basin.
FOOTNOTES:
[409-A] Maclure, Journ. de Phys., vol. lxvi. p. 219., 1808; cited by
Daubeny, Description of Volcanos, p. 24.
[410-A] This view is taken from a sketch which I made on the spot in 1830.
[416-A] Trans. of Geol. Soc., 2d series, vol. v.
[419-A] Scrope, Edin. Journ. of Sci., June, 1826, p. 145.
[419-B] Hibbert, Extinct Volcanos of the Rhine, p. 24.