The Student's Elements of Geology Part 68

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 Sub-apennine hills in such a manner as to leave no doubt that they were the products of eruptions which occurred when the sh.e.l.ly marls and sands of the Sub-appenine hills were in the course of deposition. This opinion I expressed after my visit to Italy in 1828 (See 1st edition of Principles of Geology volume 3 chapters 8 and 14 1833 and former editions of this work chapter 31.), and it has recently (1850) been confirmed by the argument adduced by Sir R. Murchison in favour of the submarine origin of the tertiary volcanic rocks of Italy. (Quarterly Geological Journal volume 6 page 281.) These rocks are well- known to rest conformably on the Sub-apennine marls, even as far south as Monte Mario, in the suburbs of Rome. On the exact age of the deposits of Monte Mario new light has recently been thrown by a careful study of their marine fossil sh.e.l.ls, undertaken by MM. Rayneval, Van den Hecke, and Ponzi. They have compared no less than 160 species with the sh.e.l.ls of the Coralline Crag of Suffolk, so well described by Mr. Searles Wood; and the specific agreement between the British and Italian fossils is so great, if we make due allowance for geographical distance and the difference of lat.i.tude, that we can have little hesitation in referring both to the same period, or to the Older Pliocene of this work. It is highly probable that, between the oldest trachytes of Tuscany and the newest rocks in the neighbourhood of Naples, a series of volcanic products might be detected of every age from the Older Pliocene to the historical epoch.

PLIOCENE VOLCANOES OF THE EIFEL.

Some of the most perfect cones and craters in Europe, not even excepting those of the district round Vesuvius, may be seen on the left or west bank of the Rhine, near Bonn and Andernach. They exhibit characters distinct from any which I have observed elsewhere, owing to the large part which the escape of aqueous vapour has played in the eruptions and the small quant.i.ties of lava emitted. 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 volcanoes 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 come 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 inward towards the mountain. When we have ascended to a considerable height, we see fragments of scoriae sparingly scattered over the surface; until at length, on reaching the summit, we find ourselves suddenly on the edge of a tarn, or deep circular lake-basin called the Gemunder Maar. In it we recognise 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 outward on every side, which we have been accustomed to consider as characteristic of volcanic vents. As we proceed, however, to the opposite side of the lake, we find a considerable quant.i.ty 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.

Other crater lakes of circular or oval form, and hollowed out of similar ancient strata, occur in the Upper Eifel, where copious aeriform discharges have taken place, throwing out vast heaps of pulverized shale into the air. I know of no other extinct volcanoes where gaseous explosions of such magnitude have been attended by the emission of so small a quant.i.ty 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 ma.s.ses, having their strata dipping outward on all sides from a central axis, as is a.s.sumed in the theory of elevation craters, alluded to in the last chapter.

I have already given (Figure 590) an example in the Eifel of a small stream of lava which issued from one of the craters of that district at Bertrich-Baden. It shows that when some of these volcanoes were in action the valleys had already been eroded to their present depth.

TRa.s.s.

The tufaceous alluvium called tra.s.s, which has covered large areas in the Eifel, 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, as is probable, this tra.s.s was formed during the period of volcanic eruptions, it may have originated in the manner of the moya of the Andes.

We may easily conceive that a similar ma.s.s might now be produced, if a copious evolution of gases should occur in one of the lake-basins. If a breach should 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 tra.s.s can be explained. The manner in which this tra.s.s conforms to the shape of the present valleys implies its comparatively modern origin, probably not dating farther back than the Pliocene Period.

CHAPTER x.x.x.

AGE OF VOLCANIC ROCKS CONTINUED.

Volcanic Rocks of the Upper Miocene Period.

Madeira.

Grand Canary.

Azores.

Lower Miocene Volcanic Rocks.

Isle of Mull.

Staffa and Antrim.

The Eifel.

Upper and Lower Miocene Volcanic Rocks of Auvergne.

Hill of Gergovia.

Eocene Volcanic Rocks of Monte Bolca.

Trap of Cretaceous Period.

Oolitic Period.

Tria.s.sic Period.

Permian Period.

Carboniferous Period.

Erect Trees buried in Volcanic Ash in the Island of Arran.

Old Red Sandstone Period.

Silurian Period.

Cambrian Period.

Laurentian Volcanic Rocks.

VOLCANIC ROCKS OF THE UPPER MIOCENE PERIOD.

MADEIRA.

The greater part of the volcanic eruptions of Madeira, as we have already seen (Chapter 29), belong to the Pliocene Period, but the most ancient of them are of Upper Miocene date, as shown by the fossil sh.e.l.ls included in the marine tuffs which have been upraised at San Vicente, in the northern part of the island, to the height of 1300 feet above the level of the sea. A similar marine and volcanic formation const.i.tutes the fundamental portion of the neighbouring island of Porto Santo, forty miles distant from Madeira, and is there elevated to an equal height, and covered, as in Madeira, with lavas of supra-marine origin.

The largest number of fossils have been collected from the tuffs and conglomerates and some beds of limestone in the island of Baixo, off the southern extremity of Porto Santo. They amount in this single locality to more than sixty in number, of which about fifty are mollusca, but many of these are only casts. Some of the sh.e.l.ls probably lived on the spot during the intervals between eruptions, and some may have been cast up into the water or air together with muddy ejections, and, falling down again, have been deposited on the bottom of the sea. The hollows in some of the fragments of vesicular lava of which the breccias and conglomerates are composed are partially filled with calc-sinter, being thus half converted into amygdaloids. Among the fossil sh.e.l.ls common to Madeira and Porto Santo, large cones, strombs, and cowries are conspicuous among the univalves, and Cardium, Spondylus, and Lithodomus among the lamellibranchiate bivalves, and among the Echinoderms the large Clypeaster called C. altus, an extinct European Miocene fossil.

The largest list of fossils has been published by Mr. Karl Meyer, in Hartung's "Madeira;" but in the collection made by myself, and in a still larger one formed by Mr. J. Yate Johnson, several remarkable forms not in Meyer's list occur, as, for example, Pholadomya, and a large Terebra. Mr. Johnson also found a fine specimen of Nautilus (Atruria) ziczac (Figure 211), a well-known Falunian fossil of Europe; and in the same volcanic tuff of Baixo, the Echinoderm Brisus Scillae, a living Mediterranean species, found fossil in the Miocene strata of Malta. Mr. Meyer identifies one-third of the Madeira sh.e.l.ls with known European Miocene (or Falunian) forms. The huge Strombus of San Vicente and Porto Santo, S. Italicus, is an extinct sh.e.l.l of the Sub-apennine or Older Pliocene formations. The mollusca already obtained from various localities of Madeira and Porto Santo are not less than one hundred in number, and, according to the late Dr. S.P. Woodward, rather more than a third are of species still living, but many of these are not now inhabitants of the neighbouring sea.

It has been remarked (Chapter 16), that in the Older Pliocene and Upper Miocene deposits of Europe many forms occur of a more southern aspect than those now inhabiting the nearest sea. In like manner the fossil corals, or Zoantharia, six in number, which I obtained from Madeira, of the genera Astraea, Sarcinula, Hydnophora, were p.r.o.nounced by Mr. Lonsdale to be forms foreign to the adjacent coasts, and agreeing with the fauna of a sea warmer than that now separating Madeira from the nearest part of the African coast. We learn, indeed, from the observations made in 1859, by the Reverend R.T. Lowe, that more than one-half, or fifty-three in ninety, of the marine mollusks collected by him from the sandy beach of Mogador are common British species, although Mogador is 18 1/2 degrees south of the nearest sh.o.r.es of England. The living sh.e.l.ls of Madeira and Porto Santo are in like manner those of a temperate climate, although in great part differing specifically from those of Mogador. (Linnean Proceedings Zoology 1860.)

GRAND CANARY.

In the Canaries, especially in the Grand Canary, the same marine Upper Miocene formation is found. Stratified tuffs, with intercalated conglomerates and lavas, are there seen in nearly horizontal layers in sea-cliffs about 300 feet high, near Las Palmas. Mr. Hartung and I were unable to find marine sh.e.l.ls in these tuffs at a greater elevation than 400 feet above the sea; but as the deposit to which they belong reaches to the height of 1100 feet or more in the interior, we conceive that an upheaval of at least that amount has taken place. The Clypeaster altus, Spondylus gaederopus, Pectunculus pilosus, Cardita calyculata, and several other sh.e.l.ls, serve to identify this formation with that of the Madeiras, and Ancillaria glandiformis, which is not rare, and some other fossils, remind us of the faluns of Touraine.

The sixty-two Miocene species which I collected in the Grand Canary were referred by the late Dr. S.P. Woodward to forty-seven genera, ten of which are no longer represented in the neighbouring sea, namely Corbis, an African form, Hinnites, now living in Oregon, Thecidium (T. Mediterranean, identical with the Miocene fossil of St. Juvat, in Brittany), Calyptraea, Hipponyx, Nerita, Erato, Oliva, Ancillaria, and Fasciolaria.

These tuffs of the southern sh.o.r.es of the Grand Canary, containing the Upper Miocene sh.e.l.ls, appear to be about the same age as the most ancient volcanic rocks of the island, composed of slaty diabase, phonolite, and trachyte. Over the marine lavas and tuffs trachytic and basaltic products of subaerial volcanic origin, between 4000 and 5000 feet in thickness, have been piled, the central parts of the Grand Canary reaching the height of about 6000 feet above the level of the sea. A large portion of this ma.s.s is of Pliocene date, and some of the latest lavas have been poured out since the time when the valleys were already excavated to within a few feet of their present depth.

On the whole, the rocks of the Grand Canary, an island of a nearly circular shape, and 6 1/2 geographical miles diameter, exhibit proofs of a long series of eruptions beginning like those of Madeira, Porto Santo, and the Azores, in the Upper Miocene period, and continued to the Post-Pliocene. The building up of the Grand Canary by subaerial eruptions, several thousand feet thick, went on simultaneously with the gradual upheaval of the earliest products of submarine eruptions, in the same manner as the Pliocene marine strata of the oldest parts of Vesuvius and Etna have been upraised during eruptions of Post-tertiary date.

In proof that movements of elevation have actually continued down to Post- tertiary times, I may remark that I found raised beaches containing sh.e.l.ls of the Recent Period in the Grand Canary, Teneriffe, and Porto Santo. The most remarkable raised beach which I observed in the Grand Canary, in the study of which I was a.s.sisted by Don Pedro Maffiotte, is situated in the north-eastern part of the island at San Catalina, about a quarter of a mile north of Las Palmas. It intervenes between the base of the high cliff formed of the tuffs with Miocene sh.e.l.ls and the sea-sh.o.r.e. From this beach, at an elevation of twenty-five feet above high-water mark, and at a distance of about 150 feet from the present sh.o.r.e, I obtained more than fifty species of living marine sh.e.l.ls.

Many of them, according to Dr. S.P. Woodward, are no longer inhabitants of the contiguous sea, as, for example, Strombus bubonius, which is still living on the West Coast of Africa, and Cerithium procerum, found at Mozambique; others are Mediterranean species, as Pecten Jacobaeus and P. polymorphus. Some of these testacea, such as Cardita squamosa, are inhabitants of deep water, and the deposit on the whole seems to indicate a depth of water exceeding a hundred feet.

AZORES.

In the island of St. Mary's, one of the Azores, marine fossil sh.e.l.ls have long been known. They are found on the north-east coast on a small projecting promontory called Ponta do Papagaio (or Point-Parrot), chiefly in a limestone about twenty feet thick, which rests upon, and is again covered by, basaltic lavas, scoriae, and conglomerates. The pebbles in the conglomerate are cemented together with carbonate of lime.

Mr. Hartung, in his account of the Azores, published in 1860, describes twenty- three sh.e.l.ls from St. Mary's (Hartung Die Azoren 1860 also Insel Gran Canaria, Madeira und Porto Santo 1864 Leipsig.), of which eight perhaps are identical with living species, and twelve are with more or less certainty referred to European Tertiary forms, chiefly Upper Miocene. One of the most characteristic and abundant of the new species, Cardium Hartungi, not known as fossil in Europe, is very common in Porto Santo and Baixo, and serves to connect the Miocene fauna of the Azores and the Madeiras. In some of the Azores, as well as in the Canary islands, the volcanic fires are not yet extinct, as the recorded eruptions of Lanzerote, Teneriffe, Palma, St. Michael's, and others, attest.

LOWER MIOCENE VOLCANIC ROCKS.

ISLE OF MULL AND ANTRIM.

I may refer the reader to the account already given (Chapter 15) of leaf-beds at Ardtun, in the Isle of Mull in the Hebrides, which bear a relation to the a.s.sociated volcanic rocks of Lower Miocene date a.n.a.logous to that which the Madeira leaf-bed, above described (Chapter 29), bears to the Pliocene lavas of that island. Mr. Geikie has shown that the volcanic rocks in Mull are above 3000 feet in thickness. There seems little doubt that the well-known columnar basalt of Staffa, as well as that of Antrim in Ireland, are of the same age, and not of higher antiquity, as once suspected.

THE EIFEL.

A large portion of the volcanic rocks of the Lower Rhine and the Eifel are coeval with the Lower Miocene deposits to which most of the "Brown-Coal" of Germany belongs. The Tertiary strata of that age are 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. The Brown-Coal formation of that region consists of beds of loose sand, sandstone, and conglomerate, clay with nodules of clay-iron-stone, 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. 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 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 feldspar. Trachytic tuff is also very abundant.

M. Von Dechen, in his work on the Siebengebirge, has given a copious list of the animal and vegetable remains of the fresh-water strata a.s.sociated with the brown-coal of that part of Germany. (Geognost. Beschreib. des Siebengebirges am Rhein Bonn 1852.) Plants of the genera Flabellaria, Ceanothus, and Daphnogene, including D. cinnamomifolia (Figure 155), occur in these beds, with nearly 150 other plants. The fishes of the brown-coal near Bonn 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 a small number of species, some of which were referred by Aga.s.siz to the genera Leuciscus, Aspius, and Perca. The remains of frogs also, of 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 the remains of many insects.

UPPER AND LOWER MIOCENE VOLCANIC ROCKS OF AUVERGNE.

The extinct volcanoes of Auvergne and Cantal, in central France, seem to have commenced their eruptions in the Lower Miocene period, but to have been most active during the Upper Miocene and Pliocene eras. I have already alluded to the grand succession of events of which there is evidence in Auvergne since the last retreat of the sea (see Chapter 29).

The earliest monuments of the Tertiary Period in that region are lacustrine deposits of great thickness, in the lowest conglomerates of which are rounded pebbles of quartz, mica-schist, granite, and other non-volcanic rocks, without the slightest intermixture of igneous products. To these conglomerates succeed argillaceous and calcareous marls and limestones, containing Lower Miocene sh.e.l.ls and bones of mammalia, the higher beds of which sometimes alternate with volcanic tuff of contemporaneous origin. After the filling up or drainage of the ancient lakes, huge piles of trachytic and basaltic rocks, with volcanic breccias, acc.u.mulated to a thickness of several thousand feet, and were superimposed upon granite, or the contiguous lacustrine strata. The greater portion of these igneous rocks appear to have originated during the Upper Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.

In Auvergne, the most ancient and conspicuous of the volcanic ma.s.ses is Mont Dor, which rests immediately on the granitic rocks standing apart from the fresh-water strata. This great mountain rises suddenly to the height of several thousand feet above the surrounding platform, and retains the shape of a flattened and somewhat irregular cone, the slope of which is gradually lost in the high plain around. This cone is composed of layers of scoriae, pumice- stones, and their fine detritus, with interposed beds of trachyte and basalt, which descend often in uninterrupted sheets until they reach and spread themselves round the base of the mountain. (Scrope Central France page 98.) Conglomerates, also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various ma.s.ses are seen to dip off from the central axis, and to lie parallel to the sloping flanks of the mountain. The summit of Mont Dor terminates in seven or eight rocky peaks, where no regular crater can now be traced, but where we may easily imagine one to have existed, which may have been shattered by earthquakes, and have suffered degradation by aqueous agents. Originally, perhaps, like the highest crater of Etna, it may have formed an insignificant feature in the great pile, and, like it, may frequently have been destroyed and renovated.

Respecting the age of the great ma.s.s of Mont Dor, we can not come at present to any positive decision, because no organic remains have yet been found in the tuffs, except impressions of the leaves of trees of species not yet determined.

It has already been stated (Chapter 15) that the earliest eruptions must have been posterior in origin to those grits and conglomerates of the fresh-water formation of the Limagne which contain no pebbles of volcanic rocks. But there is evidence at a few points, as in the hill of Gergovia, presently to be mentioned, that some eruptions took place before the great lakes were drained, while others occurred after the desiccation of those lakes, and when deep valleys had already been excavated through fresh-water strata.

The valley in which the cone of Tartaret, above-mentioned (Chapter 29), is situated affords an impressive monument of the very different dates at which the igneous eruptions of Auvergne have happened; for while the cone itself is of Post-Pliocene date, the valley is bounded by lofty precipices composed of sheets of ancient columnar trachyte and basalt, which once flowed from the summit of Mont Dor in some part of the Miocene period. These Miocene lavas had acc.u.mulated to a thickness of nearly 1000 feet before the ravine was cut down to the level of the river Couze, a river which was at length dammed up by the modern cone and the upper part of its course transformed into a lake.