No. I.

VOL. V.

This volume, as the preface informs us, is but an enlarged edition of the Memoir on the Geology of Central France, published in 1826. Mr. Scrope, on a second visit to the scene of his researches, made in 1857, found his previous conclusions strengthened, and his doctrines as to the volcanic formation of the Auvergne mountains confirmed. It is well known to those who have watched the progress of geological science during the last thirty years, that to Mr. Scrope belongs the honor of having first successfully opposed the dogmas of Werner and his school on the aqueous origin of many of the rocks now recognized as volcanic. The theory of Werner was that all formations, as at present existing on the planet, were depositions from some primeval ocean. If this the ory is in some respects undoubtedly sound, it erred on the other hand in

*

under-estimating or entirely overlooking the importance of volcanic action as one of the principal causes of rock formations.

Mr. Scrope commenced his investigations with the idea that the only true method of geological inquiry was to examine the laws of nature actually in force upon the surface of the globe and to apply the deductions from the phenomena observed to the consideration of the older formations. If, on investigation, the forces now in activity should be found to produce results similar to those observed in localities now at rest, it would be fair to infer that those localities had, in earlier ages, been the scenes of the operation of the same active forces. In fact, any. other inference would be unphilosophical, and not in accordance with the facts. The processes now going on are principally: I. The atmospheric phenomena in

1858.

cluding the laws of the circulation and residence of water on the exterior of the globe. II. The action of earthquakes and volcanos. And the changes produced by those agents are chiefly: 1. Changes of level. 2. The destruction of some rocks and the reproduction of others from their materials. 3. The production of rocks from the interior of the globe upon its surface. Du ring the historical ages, we know that these have been the changes uniformly produced by the forces we have named; and we can only believe the same forces to have been at work in the periods anterior to history.

Mr. Scrope considers France as divided by the parallel of 46 deg. 30 min. into two nearly equal portions, of which the northern, generally a vast plain, is not included in his investigations. The southern portion, beginning to rise from the parallel above named, attains an elevation of 3,000 feet in the Auvergne and Forèz, and in the Gevaudan and Vivarais, of 5,500 feet. Here it is cut down by the deep valley of the Rhone which, running nearly due north and south, separates this portion from the ranges east of that river, in the depart ments Drome, Isère and Hautes Alpes.

On the south-west also this high ground descends rapidly in an irregular line to the basin of the Gironde. The principal mass of this district is composed of primary rocks, chiefly granite, overlapped on all sides by secondary strata belonging to the Jurassic system. It is also deeply indented by the vallers of the upper Loire and Allier. Within the plain of the Limagne, in the valley of the Allier, occur detached basins of carboniferous sandstone, seeming to have been deposited in hollows of the original primary rocks.

An extensive series of limestone strata embraces the whole granitic platform like a frame; on its southern border, especially, these calcareous rocks assume a remarkable development, and constitute a vast elevated platform sloping from the primary range towards the southwest. The stratification being nearly horizontal, though dipping to the south-west, this formation exhibits a series of flat-topped hills, bounded by perpendicular cliffs 600 or 800 feet high.

These plateaux have a singularly dreary and desert aspect from the monotony of their form, and their barren and rocky character. The valleys which separate them are rarely of any great width; for the most part they are winding, narrow, and all but impassable clefts.

The volcanic formations of Central France attain an elevation much greater than that of the highest parts of the granitic platform. These formations have been described as of two classes, ancient and modern, according as they seem to have been produced before or after some supposed epoch of a diluvial character, to which the excavation of the existing valleys of the district was attributed. Mr. Scrope denies that these valleys are owing to any such cause, and supposes that the process of formation has been going on from the first appearance of the land above the sea. That the causes principally at work are rain, frost and other meteoric agents, but especially the direct fall of rain from the sky, and the wash of the superficial waters, ceaselessly engaged in sapping and mining the banks of the channels which they form for themselves. In the relative position of the plateaux of basalt and trachyte which cap so many of the hills in Auvergne at various elevations, he saw proofs that the excavation of the valleys,

as well as of the plain into which they descend, has been gradually accomplished from the earliest to the latest times, and accompanied throughout by occasional volcanic eruptions. He concludes, therefore, that no clear chronological line of separation can be drawn between the ancient and modern volcanic products.

Viewing, as a whole, the district of Auvergne, the Velay, and the Vivarais, there appear six distinct groups of volcanic rocks, viz: the Mont Dore, the Cantal, and the Mezen, each with its peculiar group; the cluster of volcanic vents of the Vivarais, which have broken out in some tributary gorges of the Ardèche; the products of more isolated vents of eruption on a zone running north west and south east from Riom to the neighborhood of Aubenas on the Ardèche; and an independent group, which was not examined by Mr. Scrope south of the Cantal, near La Guiole. The

chain of puys, (as they are called) of the Limagne d'Auvergne, and the Monts Dôine, which are the first in order of approach from Paris and the north, are first considered.

The Limagne d'Auvergne is an extensive valley-plain, about twenty miles in breadth and forty in length; its soil, with the exception of some calcareous hills, is an alluvium consisting chiefly of boulders of granitic rocks, trachyte and basalt through which the Allier still wears its channel in a course from south to north. The inclination of the surface of the plain towards the river on either side, where not interrupted by hills, averages twenty feet in a mile. The western limit of the plain is formed by the

abrupt escarpment of the granitic platform, which is fringed by some lower hills that branch off into the plain, and furrowed by deep and short ravines.

These ravines terminate at the base of the range of volcanie hills or puys, which rise from the nearly level plateau in a line almost due north and south. On the western side of this chain of puys the platform slopes towards the Sioule, which runs nearly parallel with the chain. The width of this granitic table land is about twelve miles; its average elevation 2,800 feet, being about 1,600 feet above the plain of the Limagne, but in places. where it has been preserved from denudation by a capping of basalt, it attains an elevation of 3,300 feet. On the western side the platform is composed of gneiss, but on the east of veined granite, varying frequently from a coarse to an extremely fine grain. Every storm washes away heaps of crystalline sand from the exposed surfaces of this rock. The chain of puys on this platform numbers about seventy volcanic hills of various sizes, sometimes grouped together in immediate contact, sometimes with considerable distance between them; the whole forming a notched and irregular ridge directed north and south, about twenty miles in length, by two in breadth. With the exception of five, (among which is the Puy de Dôme, the loftiest of these hills) the puys are volcanic cones of eruption,* seemingly of recent production. Their height is from 500 to 1,000 feet above their base. They are generally clothed with coarse herbage or heather; some few with thick

woods of beach. Many considera ble portions appear to have been always bare of vegetation. They appear entirely and uniformly composed of loose scoriæ, blocks of lava, and puzzolana, with occasional fragments of domite and granite. The crater is often perfect, and the hill must then be mounted to observe it; but frequently it is found broken down on the side whence the lava issued. The volcano sometimes evidently continued to eject scoria and ashes after the lava had ceased to flow-a circumstance often remarked in the eruptions of Etna. Sometimes, as it would seem, (and this is common to the eruptions of most recent volcanos) the lava has been produced by one orifice, while the aëriform jets issued from another, the latter presenting a complete cone of scoriæ and fragments, the former a broken and imperfect one. The lava has flowed either to the east or west, according to the level of the ground; the larger number of currents towards the plain of the Limagne, but some on the side of the Sioule; and these latter are more conspicuous from the gentleness of the slope on that side.

Although all the cones of the chain of puys may be considered of recent formation, they do not belong to a single epoch. The different aspects of their lava currents, some of which have yielded considerably to decomposition, while others are still bare, harsh, and uninjured, might not indeed seem conclusive as to the comparative age of the eruption; since the power of time in decomposing the surface of the lava varies according to the mine

rals which enter into its constitution. But the considerable dilapidation of some cones, and the elevated position of their currents relatively to the surrounding soil are strong indications of superior antiquity, particularly when coinciding with the testimony afforded by the condition of the lava currents. Although comparatively recent, the eruptions of these cones must have occurred previous to the earliest records of the locality, in which no mention is made of eruptions.

In the middle of this line of puys rises the celebrated Puy de Dôme; far superior in bulk and elevation to the numerous hills which stretch from its base north and south. Its height above the sea is 4,842 feet, and about 1,600 feet above its base, the sides sloping at an angle of from 30 deg. to 60 deg. It consists entirely of the variety of trachyte, which has been named Domite.

This mountain, with four neighboring hills of much less size composed of the same rock, are so closely connected in situation with some of the volcanic cones as to leave no doubt of their having been produced at the same time, and by the same volcanic agency. Each one of these hills is entirely composed of the trachyte above mentioned, without traces of definite structure. The substance of one differs only in accidental characters from that of another. The color of the rock is generally greyish or brownish white; it absorbs moisture with avidity, and the action is accompanied by a hissing noise, and a disengagement of air-bubbles. The rock is extremely liable to decomposition, which affects it often