CHAPTER XXXII.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS--_continued_.

Volcanic rocks of the Pliocene and Miocene periods continued--Auvergne--Mont Dor--Breccias and alluviums of Mont Perrier, with bones of quadrupeds--River dammed up by lava-current--Range of minor cones from Auvergne to the Vivarais--Monts Dome--Puy de Come--Puy de Pariou--Cones not denuded by general flood--Velay--Bones of quadrupeds buried in scoriae--Cantal--Eocene volcanic rocks--Tuffs near Clermont--Hill of Gergovia--Trap of Cretaceous period--Oolitic period--New Red Sandstone period--Carboniferous period--Old Red Sandstone period--"Rock and Spindle" near St. Andrews--Silurian period--Cambrian volcanic rocks.

_Tertiary Volcanic Rocks.--Auvergne._--The extinct volcanos of Auvergne and Cantal in Central France seem to have commenced their eruptions in the Upper Eocene period, but to have been most active during the 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 p. 178.).

The earliest monuments of the tertiary period in that region are lacustrine deposits of great thickness (2. fig. 480. p. 424.), 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 (3. fig. 480.) containing Upper Eocene shells 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, accumulated 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 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 masses is Mont Dor, which rests immediately on the granitic rocks standing apart from the freshwater strata.[422-A] 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, all the sides sloping more or less rapidly, until their inclination 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, till they reach and spread themselves round the base of the mountain.[423-A] Conglomerates, also, composed of angular and rounded fragments of igneous rocks, are observed to alternate with the above; and the various masses 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 may frequently have been destroyed and renovated.

According to some geologists, this mountain, as well as Vesuvius, Etna, and all large volcanos, has derived its dome-like form not from the preponderance of eruptions from one or more central points, but from the upheaval of horizontal beds of lava and scoriae. I have explained my reasons for objecting to this view at the close of Chap. XXIX., when speaking of Palma, and in the Principles of Geology.[423-B] The average inclination of the dome-shaped mass of Mont Dor is 8 deg. 6', whereas in Mounts Loa and Kea, before mentioned, in the Sandwich Islands (see fig. 457. p. 394.), the flanks of which have been raised by recent lavas, we find from Mr. Dana's description that the one has a slope of 6 deg. 30', the other of 7 deg. 46'. We may, therefore, reasonably question whether there is any absolute necessity for supposing that the basaltic currents of the ancient French volcano were at first more horizontal than they are now. Nevertheless it is highly probable that during the long series of eruptions required to give rise to so vast a pile of volcanic matter, which is thickest at the summit or centre of the dome, some dislocation and upheaval took place; and during the distension of the mass, beds of lava and scoriae may, in some places, have acquired a greater, in others a less inclination, than that which at first belonged to them.

Respecting the age of the great mass of Mont Dor, we cannot 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. We may certainly conclude, that the earliest eruptions were posterior in origin to those grits, and conglomerates of the freshwater formation of the Limagne, which contain no pebbles of volcanic rocks; while, on the other hand, some eruptions took place before the great lakes were drained; and others occurred after the desiccation of those lakes, and when deep valleys had already been excavated through freshwater strata.

In the annexed section, I have endeavoured to explain the geological structure of a portion of Auvergne, which I re-examined in 1843.[423-C] It may convey some idea to the reader of the long and complicated series of events, which have occurred in that country, since the first lacustrine strata (No. 2.) were deposited on the granite (No. 1.). The changes of which we have evidence are the more striking, because they imply great denudation, without there being any proofs of the intervention of the sea during the whole period. It will be seen that the upper freshwater beds (No. 3.), once formed in a lake, must have suffered great destruction before the excavation of the valleys of the Couze and Allier had begun. In these freshwater beds, Upper Eocene fossils, as described in Chap. XV., have been found. The basaltic dike 4' is one of many examples of the intrusion of volcanic matter through the Eocene freshwater beds, and may have been of Upper Eocene or Miocene date, giving rise, when it reached the surface and overflowed, to such platforms of basalt, as often cap the tertiary hills in Auvergne, and one of which (4) is seen on Mont Perrier.

It not unfrequently happens that beds of gravel containing bones of extinct mammalia are detected under these very ancient sheets of basalt, as between No. 4. and the freshwater strata, No. 3., at A, from which it is clear that the surface of 3 formed at that period the lowest level at which the waters then draining the country flowed. Next in age to this basaltic platform comes a patch of ochreous sand and gravel (No. 5.), containing many bones of quadrupeds. Upon this rests a pumiceous breccia and conglomerate, with angular masses of trachyte, and some quartz pebbles. This deposit is followed by 5 _b_, which is similar to 5, and 5 _c_ similar to the trachytic breccia 5 _a_. These two breccias are supposed, from their similarity to others found on Mount Dor, to have descended from the flanks of that mountain during eruptions; and the interstratified alluvial deposits contain the remains of mastodon, rhinoceros, tapir, deer, beaver, and quadrupeds of other genera referable to about forty species, all of which are extinct. I formerly supposed them to belong to the same era as the Miocene faluns of Touraine; but, whether they may not rather be ascribed to the older Pliocene epoch is a question which farther inquiries and comparisons must determine.

Whatever be their date in the tertiary series, they are quadrupeds which inhabited the country when the formations 5 and 5 _c_ originated. Probably they were drowned during floods, such as rush down the flanks of volcanos during eruptions, when great bodies of steam are emitted from the crater, or when, as we have seen, both on Etna and in Iceland in modern times, large masses of snow are suddenly melted by lava, causing a deluge of water to bear down fragments of igneous rocks mixed with mud, to the valleys and plains below.

It will be seen that the valley of the Issoire, down which these ancient inundations swept, was first excavated at the expense of the formations 2, 3, and 4, and then filled up by the masses 5 and 5 _c_, after which it was re-excavated before the more modern alluviums (Nos. 6. and 7.) were formed. In these again other fossil mammalia of distinct species have been detected by M. Bravard, the bones of an hippopotamus having been found among the rest.

At length, when the valley of the Allier was eroded at Issoire down to its lowest level, a talus of angular fragments of basalt and freshwater limestone (No. 8.) was formed, called the bone-bed of the Tour de Boulade, from which a great many other mammalia have been collected by MM. Bravard and Pomel. In this assemblage the _Elephas primigenius_, _Rhinoceros tichorinus_, _Deer_ (including rein-deer), _Equus_, _Bos_, _Antelope_, _Felis_, and _Canis_, were included. Even this deposit seems hardly to be the newest in the neighbourhood, for if we cross from the town of Issoire (see fig. 480.) over Mont Perrier to the adjoining valley of the Couze, we find another bone-bed (No. 9.), overlaid by a current of lava (No. 10.).

The history of this lava-current, which terminates a few hundred yards below the point No. 10., in the suburbs of the village of Nechers, is interesting. It forms a long narrow stripe more than 13 miles in length, at the bottom of the valley of the Couze, which flows out of a lake at the foot of Mont Dor. This lake is caused by a barrier thrown across the ancient channel of the Couze, consisting partly of the volcanic cone called the Puy de Tartaret, formed of loose scoriae, from the base of which has issued the lava-current before mentioned. The materials of the dam which blocked up the river, and caused the Lac de Chambon, are also, in part, derived from a land-slip which may have happened at the time of the great eruption which formed the cone.

This cone of Tartaret affords an impressive monument of the very different dates at which the igneous eruptions of Auvergne have happened; for it was evidently thrown up at the bottom of the existing valley, which is bounded by lofty precipices composed of sheets of ancient columnar trachyte and basalt, which once flowed at very high levels from Mont Dor.[425-A]

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 mass 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 passed 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 associated with the fossils before enumerated, and several recent land shells, 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 principally 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 deg., 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 illustration of the same phenomenon in the Puy Rouge, a conical hill to the north of the village of Pranal. 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 mass 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 principal 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 masses 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 mass, having an average slope of only 4 deg., which has evidently been accumulated, 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 accumulated in a depression of a tract composed of micaceous schist. In the breccias, even to the very summit of the mountain, we find ejected masses of the freshwater beds, and sometimes fragments of flint, containing Eocene shells. 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 shells, 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. Shifts 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 massive 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 shells. 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 associated tuffs, prove them to have been derived from volcanic eruptions which occurred during the deposition of the lacustrine strata.

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 shells, 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 shells 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 mass, 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 class 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 masses, called ophiolites in the Apennines, and associated 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 Devonshire, trappean rocks are associated 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 classes 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 shores of the Forth, in Fifeshire, 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 class of carboniferous traps are traced along the south margin of Stratheden, and constitute 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 associated 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 masses 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 mass 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 circumference (or tire, as it were, of the wheel), as in the accompanying figure. I conceive this mass 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.).

A trap dike was pointed out to me by Dr. Fleming, in the parish of Flisk, in the northern part of Fifeshire, which cuts through the grey sandstone and shale, forming the lowest part of the Old Red Sandstone. It may be traced for many miles, passing through the amygdaloidal and other traps of the hill called Normans Law. In its course it affords a good exemplification of the passage 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 quantity of magnetic iron, perhaps titaniferous, is also present. The result of this analysis is interesting, because both the ancient and modern lavas of Etna consist in like manner of augite, Labradorite, and titaniferous iron.

_Trap of the Old Red sandstone period._--By referring to the section explanatory of the structure of Forfarshire, 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 masses 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 massive 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 Shropshire, that when the lower Silurian strata of that county were accumulating, 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 Shropshire and Montgomeryshire, 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 associated sandstone, limestone, and shale, and having the same strike and dip.[435-B]

In Radnorshire 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 Cumberland, 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 passage 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.

[425-A] For a view of Puy de Tartaret and Mont Dor, see Scrope's Volcanos of Central France.

[427-A] Scrope's Central France, p. 60., and plate.

[428-A] Daubeny on Volcanos, p. 14.

[428-B] Edin. Journ. of Sci., No. iv. N. S. p. 276. Figures of some of these remains are given by M. Bertrand de Doue, Ann. De la Soc. d'Agricult. de Puy, 1828.

[429-A] Mem. de la Soc. Geol. de France, tom. i. p. 175.

[429-B] See Lyell and Murchison, Ann. de Sci. Nat., Oct. 1829.

[430-A] See Scrope's Central France, p. 21.

[430-B] Ibid, p. 7.

[431-A] Boblaye and Virlet, Morea, p. 23.

[432-A] De la Beche, Geol. Proceedings, No. 41. p. 196.

[432-B] "The rock," as English readers of Burn's poems may remember, is a Scotch term for distaff.

[435-A] Murchison, Silurian System, &c. p. 230.

[435-B] Ibid., p. 272.

[435-C] Ibid., p. 325.

[435-D] Chap. XXVII. p. 356.

[435-E] Geol. Trans., 2d series, vol. iv. p. 55.