A Manual of Elementary Geology Part 64

_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 quant.i.ty 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 Francis...o...b..los, 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 pa.s.sed through the valley since the scoriae fell, or these would have been for the most part removed.

[Ill.u.s.tration: 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 pa.s.sing 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 const.i.tute 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 a.s.sumes 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 ma.s.s 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 superinc.u.mbent 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 const.i.tuted 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.

[Ill.u.s.tration: 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 ma.s.s 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 const.i.tuted 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, pa.s.sing 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 ma.s.s 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 circ.u.mstance 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 Francis...o...b..los 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.

[Ill.u.s.tration: 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 sh.e.l.ls 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 s.p.a.ce 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 princ.i.p.ally 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.

[Ill.u.s.tration: 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 acc.u.mulation 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 princ.i.p.ally 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. Aga.s.siz 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 sh.e.l.ls 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 sh.e.l.ls, 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 sh.e.l.ls 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 coa.r.s.e-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 a.s.sociated 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.

[Ill.u.s.tration: Fig. 477. The Gemunder Maar.]

[Ill.u.s.tration: Fig. 478. Cross section.

_a._ Village of Gemund.

_b._ Gemunder Maar.