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This volume, as the preface in- under-estimating or entirely overforms us, is but an enlarged edition looking the importance of volcanic of the Memoir on the Geology of action as one of the principal causes Central France, published in 1826. of rock formations. Mr. Scrope, on a second visit to the Mr. Serope commenced his invesscene of his researches, made in tigations with the idea that the only 1857, found his previous conclu- true method of geological inquiry sions strengthened, and his doc- was to examine the laws of nature trines as to the volcanic formation actually in force upon the surface of the Auvergne mountains con- of the globe and to apply the defirmed. It is well known to those ductions from the phenomena obwho have watched the progress of served to the consideration of the geological science during the last older formations. If, on investigathirty years, that to Mr. Scrope be- tion, the forces now in activity longs the honos of having first suc- should be found to produce results cessfully opposed the dogmas of similar to those observed in localiWerner and his school on the ties now at rest, it would be fair to aqueous origin of many of the rocks infer that those localities had, in now recognized as volcanic. The earlier ages, been the scenes of the theory of Werner was that all for- operation of the same active forces. mations, as at present existing on In fact, any. other inference would the planet, were depositions from be unpbilosophical, and not in acsome primeval ocean. If this the- cordance with the facts. The pro. ory is in some respects undoubtedly cesses now going on are principally: sound, it erred on the other hand in I. The atmospheric phenomena in
*The geology and extinct Volcanos of Central France. By G. Poulett Scrope, M. P., F. R. S., F.G. S., &c. Second Edition, enlarged and improved, with illustrative Maps, Views and Panoramic sketches. London: John Murray, Albemarle street. 1858. VOL. V.
cluding the laws of the circulation An extensive series of limestone and residence of water on the exte. strata embrares the whole granitic rior of the globe. II. The action platform like a frame; on its southof earthquakes and volcanos. And ern border, especially, these calcathe changes produced by those reous rocks assume a remarkable agents are chiefly: 1. Changes of development, and constitute a vast level. 2. The destruction of some elevated platform sloping from the rocks and the reproduction of others primary range towards the southfrom their materials. 3. The pro- west. The stratification being near. duction of rocks from the interior ly horizontal, though dipping to the of the globe upon its surface. Du south-west, this formation exhibits ring the historical ages, we know a series of flat-topped hills, bounded that these have been the changes by perpendicular clitfs 600 or 800 uniformly produced by the forces feet high. we have named ; and we can only These plateaux have a singularly believe the same forces to have been dreary and desert aspect from the at work in the periods anterior to monotony of their form, and their history:
barreni and rocky character. The Mr. Serope considers France as valleys winich separate them are divi.led by the parallel of 46 deg. rarely of any great width; for the 30 min. into two nearly equal por- most part they are winding, narrow, tions, of which the northern, gen- and all but impassable clefts. erally a vast plain, is not included The volcanic furinations of Cenin his investigations. The southern tral France attain an elevation portion, beginning to rise from the much greater than that of the highparallel above named, attains an est parts of the granitic platform. elevation of 3,000 feet in the Au. These formations have been devergne and Forèz, and in the Gevau- scribed as of two classes, ancient dan and Vivarais, of 5,500 feet. anil modern, according as they seem Here it is cut down by the deep to have been produred before or valley of the Rhone which, running after some supposed epocht of a nearly due north and south, separ- diluvial character, to which the exates this portion from the ranges cavation of the existing valleys of east of that river, in the depart. the district was attributerl.
Mr. ments Drome, Isère and lautes Scrope denies that these vallers aro Alpes.
owing to any suroli cause, and
supOn the south-west also this high poses that the process of formation grounil descends rapidly in an ir- lias been going on from the first regular line to the basin of the appearance of the land above the Gironde. The principal mass of sea. That the causes principally at this district is composed of primary work are rain, frost and other inerocks, chietly granite, overlapped teoric agents, but esperially the on all sides by secondary strata be- direct full of rain from the sky, and longing to the Jurassic system. It the wash of the superficial waters, is also deeply indented by the val- ceaselessly engaged in sapping and lers of the upper Loire and Allier. mining the banks of the channels Within the plain of the Limagne, which they form for themselves. In in the valley of the Allier, occur the relative position of the plateaux deta-hed basins of carboniferous of basalt and trachyte which cap so sandstone, seeming to have been inany of the hills in Auveryne at depositel in hollows of the original various elevations, he saw proofs
that the excavation of the valleys,
as well as of the plain into which abrupt escarpment of the granitic they descend, has been gradually platform, which is fringed by soine accomplished from the earliest to lower hills that branch off into the the latest times, and accompanied plain, and furrowed by deep and throughout hy occasional volcanic short ravines. eruptions. He concludes, therefore, These ravines terminate at the that no clear chronological line of base of the range of volcanie hills separation can be drawn between or puys, which rise from the nearly the ancient and modern volcanic level plateau in a line almost due products.
north and south. On the western Viewing, as a whole, the district side of this chain of puys the platof Auvergne, the Velay, and the form slopes towards the Sioule, Vivarais, i here
six distinct which runs nearly parallel with the groups of volcanic rocks, viz: the chain. The width of this granitic Mont Dore, the Cantal, and the table land is about twelve miles; Mezen, each with its peculiar group; its average elevation 2,300 feet, the cluster of volcanic vents of the being about 1,600 feet above the Vivarais, which have broken out in plain of the Limagne, but in places some tributary gorges of the Ar- where it has been preserved from dèche; the products of more isola- denudation by a capping of basalt, ted vents of eruption on a zone it attains an elevation of 3,300 feet. running north-west and south east On the western side the platform is from Riom to the neighborhood of composer of gneiss, but on the east Aubenas on the Ardècbe; and an of veined granite, varying frequentindependent group, whiųh was not ly from a coarse to an extremely examined by Mr. Serope south of fine grain. Every storm washes the Cantal, near La Guiole. The away heaps of crystalline sand from chain of puys, (as they are called) the exposed surfaces of this rock. of the Limagne d'Auvergne, and The chain of puys on this platform the Monts Dôine, which are the first numbers about seventy volcanic in order of approach from Paris and hills of various sizes, sometimes the north, are first considered. grouped together in immediate
The Limagne d'Auvergne is an contact, sometimes with consideraextensive valley.plain, about twenty ble distance between them; the miles in breadth and forty in whole forming a notched and irlength; its soil, with the exception regular ridge directed north and of some calcareous hills, is an allu- south, about twenty miles in length, vium consisting chiefly of boulders by two in breadth. With the ex: of granitic rocks, trachite and ception of five, (among which is basalt through which the Allier still the Puy de Dôme, the loftiest of wears its channel in a course from these bills) the puys are volcanic south to north. The inclination of cones of eruption,* seemingly of the surface of the plain towards recent production. Their height the river on either side, where not is from 500 to 1,000 feet above interrupted by bills, averages twen- their base. They are generally ly feet in a mile. The western clothed with coarse herbage or limit of the plain is formeil by the heather; some few with thick
*A volcanic “cone of eruption" in its normal form, with a crater or cup-shaped hollow at its summit, is the result of the accumulation round the volcanic orifice or vent of the scoria and other fragınentary matters projected into the air by the series of explosive discharges of elastic vapor and gases which usually characterises an eruption. The fragments which fall back into the vent are, of course,
woods of beach. Many considera rals which enterinto its constitution. ble portions appear to have been But the considerable dilapidation always bare of vegetation. They of some cones, and the elevated appear entirely and uniformly com- position of their currents relatively posed of loose scoriæ, blocks of lava, to the surrounding soil are strong and puzzolana, with occasional frag- indications of superior antiquity, ments of domite and granite. The particularly when coinciding with crater is often perfect, and the hill the testimony afforded by the conmust then be mounted to observe dition of the lava currents. Alit; but frequently it is found broken though comparatively recent, the down on the side whence the lava eruptions of these cones must have issued. The volcano sometimes occurred previous to the earliest evidently continued to eject scoriæ records of the locality, in which no and ashes after the lava had ceased mention is made of eruptions. to flow-a circumstance often re In the middle of this line of puys marked in the eruptions of Etna. rises the celebrated Puy de Dôme; Sometimes, as it would seem, (and far superior in bulk and elevation this is common to the eruptions of to the numerous hills which stretch most recent volcanos) the lava bas from its base north and south. Its been produced by one orifice, while height above the sea is 4,842 feet, the aëriform jets issued from an- apd about 1,600 feet above its base, other, the latter presenting a com- the sides sloping at an angle of plete cone of scoriæ and fragments, from 30 deg. to 60 deg. It consists the former a broken and imperfect entirely of the variety of trachyte,
The lava has flowed either to which has been named Domite. the east or west, according to the This mountain, with four neighlevel of the ground; the larger boring hills of much less size comnumber of currents towards the posed of the same rock, are so plain of the Limagne, but some on closely connected in situation with the side of the Sioule; and these some of the volcanic codes as to latter are more conspicuous from leave no doubt of their having been the gentleness of the slope on that produced at the same time, and by side.
the same volcanic agency. Each Although all the cones of the one of these bills is entirely comchain of puys may be considered of posed of the trachyte above menrecent forination, they do not belong tioned, without traces of definite to a single epoch. The different structure. The substance of one aspects of their lava currents, some differs only in accidental characters of which have yielded considerably from that of another. The color to decomposition, while others are of the rock is generally greyish or still bare, harsh, and uninjured, brownish white; it absorbs moismight not indeed seem conclusive as ture with avidity, and the action is to the comparative age of the erup- accompanied by a hissing noise, tion; since the power of time in and a disengagement of air-bubbles. decomposing the surface of the The rock is extremely liable to lava varies according to the mine- decomposition, which affects it often
thrown up again and again, and triturated into gravelly sand or fine ashes by the friction attendant on this violent process. Those which fall on the outside of the vent are heaped up there in a circular bank, the sides of which, both within and without, slope at an angle rarely exceeding 33 deg. And this bank, viewed externally, has of course the shape of a truncated cone, the crater being a hollow inverted cone contained within it.