A Manual of Elementary Geology Part 66

When we follow the course of the river Couze, from its source in the lake of Chambon, to the termination of the lava-current at Nechers, a distance of thirteen miles, we find that the torrent has in most places cut a deep channel through the lava, the lower portion of which is columnar. In some narrow gorges it has even had power to remove the entire ma.s.s of basaltic rock, though the work of erosion must have been very slow, as the basalt is tough and hard, and one column after another must have been undermined and reduced to pebbles, and then to sand.

During the time required for this operation, the perishable cone of Tartaret, composed of sand and ashes, has stood uninjured, proving that no great flood or deluge can have pa.s.sed over this region in the interval between the eruption of Tartaret and our own times.

If we now return to the section (fig. 480.), I may observe that the lava-current of Tartaret, which has diminished greatly in height and volume near its termination, presents here a steep and perpendicular face 25 feet in height towards the river. Beneath it is the alluvium No.

9., consisting of a red sandy clay, which must have covered the bottom of the valley when the current of melted rock flowed down. The bones found in this alluvium, which I obtained myself, consisted of a species of field-mouse, _Arvicola_, and the molar tooth of an extinct horse, _Equus fossilis_. The other species, obtained from the same bed, are referable to the genera _Sus_, _Bos_, _Cervus_, _Felis_, _Canis_, _Martes_, _Talpa_, _Sorex_, _Lepus_, _Sciurus_, _Mus_, and _Lagomys_, in all no less than forty-three species, all closely allied to recent animals, yet nearly all of them, according to M. Bravard, showing some points of difference, like those which Mr. Owen discovered in the case of the horse above alluded to. The bones, also, of a frog, snake, and lizard, and of several birds, were a.s.sociated with the fossils before enumerated, and several recent land sh.e.l.ls, such as _Cyclostoma elegans_, _Helix hortensis_, _H. nemoralis_, _H. lapicida_, and _Clausilia rugosa_. If the animals were drowned by floods, which accompanied the eruptions of the Puy de Tartaret, they would give an exceedingly modern geological date to that event, which must, in that case, have belonged to the Newer-Pliocene, or, perhaps, the Post-Pliocene period. That the current, which has issued from the Puy de Tartaret, may nevertheless be very ancient in reference to the events of human history, we may conclude, not only from the divergence of the mammiferous fauna from that of our day, but from the fact that a Roman bridge of such form and construction as continued in use down to the fifth century, but which may be older, is now seen at a place about a mile and a half from St. Nectaire. This ancient bridge spans the river Couze with two arches, each about 14 feet wide. These arches spring from the lava of Tartaret, on both banks, showing that a ravine precisely like that now existing, had already been excavated by the river through that lava thirteen or fourteen centuries ago.

In Central France there are several hundred minor cones, like that of Tartaret, a great number of which, like Monte Nuovo, near Naples, may have been princ.i.p.ally due to a single eruption. Most of these cones range in a linear direction from Auvergne to the Vivarais, and they were faithfully described so early as the year 1802, by M. de Montlosier. They have given rise chiefly to currents of basaltic lava. Those of Auvergne called the Monts Dome, placed on a granitic platform, form an irregular ridge (see fig. 436.), about 18 miles in length, and 2 in breadth. They are usually truncated at the summit, where the crater is often preserved entire, the lava having issued from the base of the hill. But frequently the crater is broken down on one side, where the lava has flowed out. The hills are composed of loose scoriae, blocks of lava, lapilli, and pozzuolana, with fragments of trachyte and granite.

_Puy de Come._--The Puy de Come and its lava-current, near Clermont, may be mentioned as one of these minor volcanos. This conical hill rises from the granitic platform, at an angle of about 40, to the height of more than 900 feet. Its summit presents two distinct craters, one of them with a vertical depth of 250 feet. A stream of lava takes its rise at the western base of the hill, instead of issuing from either crater, and descends the granitic slope towards the present site of the town of Pont Gibaud. Thence it pours in a broad sheet down a steep declivity into the valley of the Sioule, filling the ancient river-channel for the distance of more than a mile. The Sioule, thus dispossessed of its bed, has worked out a fresh one between the lava and the granite of its western bank; and the excavation has disclosed, in one spot, a wall of columnar basalt about 50 feet high.[427-A]

The excavation of the ravine is still in progress, every winter some columns of basalt being undermined and carried down the channel of the river, and in the course of a few miles rolled to sand and pebbles.

Meanwhile the cone of Come remains stationary, its loose materials being protected by a dense vegetation, and the hill standing on a ridge not commanded by any higher ground whence floods of rain-water may descend.

_Puy Rouge._--At another point, farther down the course of the Sioule, we find a second ill.u.s.tration of the same phenomenon in the Puy Rouge, a conical hill to the north of the village of Pra.n.a.l. The cone is composed entirely of red and black scoriae, tuff, and volcanic bombs. On its western side there is a worn-down crater, whence a powerful stream of lava has issued, and flowed into the valley of the Sioule. The river has since excavated a ravine through the lava and subjacent gneiss, to the depth of 400 feet.

On the upper part of the precipice forming the left side of this ravine, we see a great ma.s.s of black and red scoriaceous lava; below this a thin bed of gravel, evidently an ancient river-bed, now at an elevation of 50 feet above the channel of the Sioule. The gravel again rests upon gneiss, which has been eroded to the depth of 50 feet. It is quite evident in this case, that, while the basalt was gradually undermined and carried away by the force of running water, the cone whence the lava issued escaped destruction, because it stood upon a platform of gneiss several hundred feet above the level of the valley in which the force of running water was exerted.

_Puy de Pariou._--The brim of the crater of the Puy de Pariou, near Clermont, is so sharp, and has been so little blunted by time, that it scarcely affords room to stand upon. This and other cones in an equally remarkable state of integrity have stood, I conceive uninjured, not _in spite_ of their loose porous nature, as might at first be naturally supposed, but in consequence of it. No rills can collect where all the rain is instantly absorbed by the sand and scoriae, as is remarkably the case on Etna; and nothing but a waterspout breaking directly upon the Puy de Pariou could carry away a portion of the hill, so long as it is not rent or engulphed by earthquakes.

Hence it is conceivable that even those cones which have the freshest aspect, and most perfect shape, may lay claim to very high antiquity. Dr.

Daubeny has justly observed, that had any of these volcanos been in a state of activity in the age of Julius Caesar, that general, who encamped upon the plains of Auvergne, and laid siege to its princ.i.p.al city (Gergovia, near Clermont), could hardly have failed to notice them. Had there been any record of their eruptions in the time of Pliny or Sidonius Apollinaris, the one would scarcely have omitted to make mention of it in his Natural History, nor the other to introduce some allusion to it among the descriptions of this his native province. This poet's residence was on the borders of the Lake Aidat, which owed its very existence to the damming up of a river by one of the most modern lava-currents.[428-A]

_Velay._--The observations of M. Bertrand de Doue have not yet established that any of the most ancient volcanos of Velay were in action during the Eocene period. There are beds of gravel in Velay, as in Auvergne, covered by lava at different heights above the channels of the existing rivers. In the highest and most ancient of these alluviums the pebbles are exclusively of granitic rocks; but in the newer, which are found at lower levels, and which originated when the valleys had been cut to a greater depth, an intermixture of volcanic rocks has been observed.

At St. Privat d'Allier a bed of volcanic scoriae and tuff was discovered by Dr. Hibbert, inclosed between two sheets of basaltic lava; and in this tuff were found the bones of several quadrupeds, some of them adhering to ma.s.ses of slaggy lava. Among other animals were _Rhinoceros leptorhinus_, _Hyaena spelaea_, and a species allied to the spotted hyaena of the Cape, together with four undetermined species of deer.[428-B] The manner of the occurrence of these bones reminds us of the published accounts of an eruption of Coseguina, 1835, in Central America (see p.

399.), during which hot cinders and scoriae fell and scorched to death great numbers of wild and domestic animals and birds.

_Plomb du Cantal._--In regard to the age of the igneous rocks of the Cantal, we can at present merely affirm, that they overlie the Eocene lacustrine strata of that country (see Map, p. 179.). They form a great dome-shaped ma.s.s, having an average slope of only 4, which has evidently been acc.u.mulated, like the cone of Etna, during a long series of eruptions.

It is composed of trachytic, phonolitic, and basaltic lavas, tuffs, and conglomerates, or breccias, forming a mountain several thousand feet in height. Dikes also of phonolite, trachyte, and basalt are numerous, especially in the neighbourhood of the large cavity, probably once a crater, around which the loftiest summits of the Cantal are ranged circularly, few of them, except the Plomb du Cantal, rising far above the border or ridge of this supposed crater. A pyramidal hill, called the Puy Griou, occupies the middle of the cavity.[429-A] It is clear that the volcano of the Cantal broke out precisely on the site of the lacustrine deposit before described (p. 188.), which had acc.u.mulated in a depression of a tract composed of micaceous schist. In the breccias, even to the very summit of the mountain, we find ejected ma.s.ses of the freshwater beds, and sometimes fragments of flint, containing Eocene sh.e.l.ls. Valleys radiate in all directions from the central heights of the mountain, increasing in size as they recede from those heights. Those of the Cer and Jourdanne, which are more than 20 miles in length, are of great depth, and lay open the geological structure of the mountain. No alternation of lavas with undisturbed Eocene strata has been observed, nor any tuffs containing freshwater sh.e.l.ls, although some of these tuffs include fossil remains of terrestrial plants, said to imply several distinct restorations of the vegetation of the mountain in the intervals between great eruptions. On the northern side of the Plomb du Cantal, at La Vissiere, near Murat, is a spot, pointed out on the Map (p. 179.), where freshwater limestone and marl are seen covered by a thickness of about 800 feet of volcanic rock. s.h.i.+fts are here seen in the strata of limestone and marl.[429-B]

_Eocene period._--In treating of the lacustrine deposits of Central France, in the fifteenth chapter, it was stated that, in the arenaceous and pebbly group of the lacustrine basins of Auvergne, Cantal, and Velay, no volcanic pebbles had ever been detected, although ma.s.sive piles of igneous rocks are now found in the immediate vicinity. As this observation has been confirmed by minute research, we are warranted in inferring that the volcanic eruptions had not commenced when the older subdivisions of the freshwater groups originated.

In Cantal and Velay no decisive proofs have yet been brought to light that any of the igneous outbursts happened during the deposition of the freshwater strata; but there can be no doubt that in Auvergne some volcanic explosions took place before the drainage of the lakes, and at a time when the Upper Eocene species of animals and plants still flourished. Thus, for example, at Pont du Chateau, near Clermont, a section is seen in a precipice on the right bank of the river Allier, in which beds of volcanic tuff alternate with a freshwater limestone, which is in some places pure, but in others spotted with fragments of volcanic matter, as if it were deposited while showers of sand and scoriae were projected from a neighbouring vent.[430-A]

Another example occurs in the Puy de Marmont, near Veyres, where a freshwater marl alternates with volcanic tuff containing Eocene sh.e.l.ls. The tuff or breccia in this locality is precisely such as is known to result from volcanic ashes falling into water, and subsiding together with ejected fragments of marl and other stratified rocks. These tuffs and marls are highly inclined, and traversed by a thick vein of basalt, which, as it rises in the hill, divides into two branches.

_Gergovia._--The hill of Gergovia, near Clermont, affords a third example.

I agree with MM. Dufrenoy and Jobert that there is no alternation here of a contemporaneous sheet of lava with freshwater strata, in the manner supposed by some other observers[430-B]; but the position and contents of some of the a.s.sociated tuffs, prove them to have been derived from volcanic eruptions which occurred during the deposition of the lacustrine strata.

[Ill.u.s.tration: Fig. 481. Hill of Gergovia.]

The bottom of the hill consists of slightly inclined beds of white and greenish marls, more than 300 feet in thickness, intersected by a dike of basalt, which may be studied in the ravine above the village of Merdogne.

The dike here cuts through the marly strata at a considerable angle, producing, in general, great alteration and confusion in them for some distance from the point of contact. Above the white and green marls, a series of beds of limestone and marl, containing freshwater sh.e.l.ls, are seen to alternate with volcanic tuff. In the lowest part of this division, beds of pure marl alternate with compact fissile tuff, resembling some of the subaqueous tuffs of Italy and Sicily called _peperinos_. Occasionally fragments of scoriae are visible in this rock. Still higher is seen another group of some thickness, consisting exclusively of tuff, upon which lie other marly strata intermixed with volcanic matter. Among the species of fossil sh.e.l.ls which I found in these strata were _Melania inquinata_, a _Unio_, and a _Melanopsis_, but they were not sufficient to enable me to determine with precision the age of the formation.

There are many points in Auvergne where igneous rocks have been forced by subsequent injection through clays and marly limestones, in such a manner that the whole has become blended in one confused and brecciated ma.s.s, between which and the basalt there is sometimes no very distinct line of demarcation. In the cavities of such mixed rocks we often find calcedony, and crystals of mesotype, stilbite, and arragonite. To formations of this cla.s.s may belong some of the breccias immediately adjoining the dike in the hill of Gergovia; but it cannot be contended that the volcanic sand and scoriae interstratified with the marls and limestones in the upper part of that hill were introduced, like the dike, subsequently, by intrusion from below. They must have been thrown down like sediment from water, and can only have resulted from igneous action, which was going on contemporaneously with the deposition of the lacustrine strata.

The reader will bear in mind that this conclusion agrees well with the proofs, adverted to in the fifteenth chapter, of the abundance of silex, travertin, and gypsum precipitated when the upper lacustrine strata were formed; for these rocks are such as the waters of mineral and thermal springs might generate.

_Cretaceous period._--Although we have no proof of volcanic rocks erupted in England during the deposition of the chalk and greensand, it would be an error to suppose that no theatres of igneous action existed in the cretaceous period. M. Virlet, in his account of the geology of the Morea, p. 205., has clearly shown that certain traps in Greece, called by him ophiolites, are of this date; as those, for example, which alternate conformably with cretaceous limestone and greensand between Kastri and Damala in the Morea. They consist in great part of diallage rocks and serpentine, and of an amygdaloid with calcareous kernels, and a base of serpentine.

In certain parts of the Morea, the age of these volcanic rocks is established by the following proofs: first, the lithographic limestones of the Cretaceous era are cut through by trap, and then a conglomerate occurs, at Nauplia and other places, containing in its calcareous cement many well-known fossils of the chalk and greensand, together with pebbles formed of rolled pieces of the same ophiolite, which appear in the dikes above alluded to.

_Period of Oolite and Lias._--Although the green and serpentinous trap rocks of the Morea belong chiefly to the Cretaceous era, as before mentioned, yet it seems that some eruptions of similar rocks began during the Oolitic period[431-A]; and it is probable, that a large part of the trappean ma.s.ses, called ophiolites in the Apennines, and a.s.sociated with the limestone of that chain, are of corresponding age.

That part of the volcanic rocks of the Hebrides, in our own country, originated contemporaneously with the Oolite which they traverse and overlie, has been ascertained by Prof. E. Forbes, in 1850.

_Trap of the New Red Sandstone period._--In the southern part of Devons.h.i.+re, trappean rocks are a.s.sociated with New Red Sandstone, and, according to Sir H. De la Beche, have not been intruded subsequently into the sandstone, but were produced by contemporaneous volcanic action. Some beds of grit, mingled with ordinary red marl, resemble sands ejected from a crater; and in the stratified conglomerates occurring near Tiverton are many angular fragments of trap porphyry, some of them one or two tons in weight, intermingled with pebbles of other rocks. These angular fragments were probably thrown out from volcanic vents, and fell upon sedimentary matter then in the course of deposition.[432-A]

_Carboniferous period._--Two cla.s.ses of contemporaneous trap rocks have been ascertained by Dr. Fleming to occur in the coal-field of the Forth in Scotland. The newest of these, connected with the higher series of coal-measures, is well exhibited along the sh.o.r.es of the Forth, in Fifes.h.i.+re, where they consist of basalt with olivine, amygdaloid, greenstone, wacke, and tuff. They appear to have been erupted while the sedimentary strata were in a horizontal position, and to have suffered the same dislocations which those strata have subsequently undergone. In the volcanic tuffs of this age are found not only fragments of limestone, shale, flinty slate, and sandstone, but also pieces of coal.

The other or older cla.s.s of carboniferous traps are traced along the south margin of Stratheden, and const.i.tute a ridge parallel with the Ochils, and extending from Stirling to near St. Andrews. They consist almost exclusively of greenstone, becoming, in a few instances, earthy and amygdaloidal. They are regularly interstratified with the sandstone, shale, and ironstone of the lower Coal-measures, and, on the East Lomond, with Mountain Limestone.

I examined these trap rocks in 1838, in the cliffs south of St. Andrews, where they consist in great part of stratified tuffs, which are curved, vertical, and contorted, like the a.s.sociated coal-measures. In the tuff I found fragments of carboniferous shale and limestone, and intersecting veins of greenstone. At one spot, about two miles from St. Andrews, the encroachment of the sea on the cliffs has isolated several ma.s.ses of trap, one of which (fig. 482.) is aptly called the "rock and spindle,"[432-B] for it consists of a pinnacle of tuff, which may be compared to a distaff, and near the base is a ma.s.s of columnar greenstone, in which the pillars radiate from a centre, and appear at a distance like the spokes of a wheel.

The largest diameter of this wheel is about twelve feet, and the polygonal terminations of the columns are seen round the circ.u.mference (or tire, as it were, of the wheel), as in the accompanying figure. I conceive this ma.s.s to be the extremity of a string or vein of greenstone, which penetrated the tuff. The prisms point in every direction, because they were surrounded on all sides by cooling surfaces, to which they always, arrange themselves at right angles, as before explained (p. 385.).

[Ill.u.s.tration: Fig. 482. Rock and Spindle, St. Andrews.

_a._ Unstratified tuff.

_b._ Columnar greenstone.

_c._ Stratified tuff.]

[Ill.u.s.tration: Fig. 483. Columns of Greenstone, seen endwise.]

A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in the northern part of Fifes.h.i.+re, which cuts through the grey sandstone and shale, forming the lowest part of the Old Red Sandstone. It may be traced for many miles, pa.s.sing through the amygdaloidal and other traps of the hill called Normans Law. In its course it affords a good exemplification of the pa.s.sage from the trappean into the plutonic, or highly crystalline texture. Professor Gustavus Rose, to whom I submitted specimens of this dike, finds the rock, which he calls dolerite, to consist of greenish black augite and Labrador felspar, the latter being the most abundant ingredient. A small quant.i.ty of magnetic iron, perhaps t.i.taniferous, is also present. The result of this a.n.a.lysis is interesting, because both the ancient and modern lavas of Etna consist in like manner of augite, Labradorite, and t.i.taniferous iron.

_Trap of the Old Red sandstone period._--By referring to the section explanatory of the structure of Forfars.h.i.+re, already given (p. 48.), the reader will perceive that beds of conglomerate, No. 3., occur in the middle of the Old Red sandstone system, 1, 2, 3, 4. The pebbles in these conglomerates are sometimes composed of granitic and quartz rocks, sometimes exclusively of different varieties of trap, which, although purposely omitted in the above section, are often found either intruding themselves in amorphous ma.s.ses and dikes into the old fossiliferous tilestones, No. 4., or alternating with them in conformable beds. All the different divisions of the red sandstone, 1, 2, 3, 4, are occasionally intersected by dikes, but they are very rare in Nos. 1. and 2., the upper members of the group consisting of red shale and red sandstone. These phenomena, which occur at the foot of the Grampians, are repeated in the Sidlaw Hills; and it appears that in this part of Scotland, volcanic eruptions were most frequent in the earlier part of the Old Red sandstone period.

The trap rocks alluded to consist chiefly of felspathic porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, often calcedonic, and forming beautiful agates. We meet also with claystone, clinkstone, greenstone, compact felspar, and tuff. Some of these rocks flowed as lavas over the bottom of the sea, and enveloped quartz pebbles which were lying there, so as to form conglomerates with a base of greenstone, as is seen in Lumley Den, in the Sidlaw Hills. On either side of the axis of this chain of hills (see section, p. 48.), the beds of ma.s.sive trap, and the tuffs composed of volcanic sand and ashes, dip regularly to the south-east or north-west, conformably with the shales and sandstones.

_Silurian period._--It appears from the investigations of Sir R.

Murchison in Shrops.h.i.+re, that when the lower Silurian strata of that county were acc.u.mulating, there were frequent volcanic eruptions beneath the sea; and the ashes and scoriae then ejected gave rise to a peculiar kind of tufaceous sandstone or grit, dissimilar to the other rocks of the Silurian series, and only observable in places where syenitic and other trap rocks protrude. These tuffs occur on the flanks of the Wrekin and Caer Caradoc, and contain Silurian fossils, such as casts of encrinites, trilobites, and mollusca. Although fossiliferous, the stone resembles a sandy claystone of the trap family.[435-A]

Thin layers of trap, only a few inches thick, alternate, in some parts of Shrops.h.i.+re and Montgomerys.h.i.+re, with sedimentary strata of the lower Silurian system. This trap consists of slaty porphyry and granular felspar rock, the beds being traversed by joints like those in the a.s.sociated sandstone, limestone, and shale, and having the same strike and dip.[435-B]

In Radnors.h.i.+re there is an example of twelve bands of stratified trap, alternating with Silurian schists and flagstones, in a thickness of 350 feet. The bedded traps consist of felspar-porphyry, clinkstone, and other varieties; and the interposed Llandeilo flags are of sandstone and shale, with trilobites and graptolites.[435-C]

The vast thickness of contemporaneous trappean rocks of lower Silurian date in North Wales, explored by our government surveyors, has been already alluded to.[435-D]

_Cambrian volcanic rocks._--Professor Sedgwick, in his account of the geology of c.u.mberland, has described various trap rocks which accompany the green slates of the Cambrian system, beneath all the rocks containing organic remains. Different felspathic and porphyritic rocks and greenstones occur, not only in dikes, but in conformable beds; and there is occasionally a pa.s.sage from these igneous rocks to some of the green quartzose slates. Professor Sedgwick supposes these porphyries to have originated contemporaneously with the stratified chloritic slates, the materials of the slates having been supplied, in part at least, by submarine eruptions oftentimes repeated.[435-E]

FOOTNOTES:

[422-A] See the map, p. 179.

[423-A] Scrope's Central France, p. 98.

[423-B] See chaps. xxiv., xxv., and xxvi., 7th and 8th editions.

[423-C] See Quarterly Geol. Journ., vol. ii. p. 77.