Part 63 (2/2)

[404-A] Consult the valuable memoir of M. L. A. Necker, Mem. de la Soc. de Phys. et d'Hist. Nat. de Geneve, tom. ii. part i. Nov. 1822.

[405-A] From a drawing of M. Necker, in Mem. above cited.

[405-B] Phil. Trans., vol. lxx., 1780.

CHAPTER x.x.xI.

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

Volcanic rocks of the Older Pliocene period--Tuscany--Rome--Volcanic region of Olot in Catalonia--Cones and lava-currents--Ravines and ancient gravel-beds--Jets of air called Bufadors--Age of the Catalonian volcanos--Miocene period--Brown-coal of the Eifel and contemporaneous trachytic breccias--Age of the brown-coal--Peculiar characters of the volcanos of the upper and lower Eifel--Lake craters--Tra.s.s--Hungarian volcanos.

_Older Pliocene period--Tuscany._--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 Subapennine 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 Subapennine hills were in the course of deposition.

_Catalonia._--Geologists are far from being able, as yet, to a.s.sign to each of the volcanic groups scattered over Europe a precise chronological place in the tertiary series; but I shall describe here, as probably referable to some part of the Pliocene period, a district of extinct volcanos near Olot, in the north of Spain, which is little known, and which I visited in the summer of 1830.

The whole extent of country occupied by volcanic products in Catalonia is not more than fifteen geographical miles from north to south, and about six from east to west. The vents of eruption range entirely within a narrow band running north and south; and the branches, which are represented as extending eastward in the map, are formed simply of two lava-streams--those of Castell Follit and Cellent.

[Ill.u.s.tration: Fig. 470. Volcanic district of Catalonia.]

Dr. Maclure, the American geologist, was the first who made known the existence of these volcanos[409-A]; and, according to his description, the volcanic region extended over twenty square leagues, from Amer to Ma.s.sanet.

I searched in vain in the environs of Ma.s.sanet, in the Pyrenees, for traces of a lava-current; and I can say, with confidence, that the adjoining map gives a correct view of the true area of the volcanic action.

_Geological structure of the district._--The eruptions have burst entirely through fossiliferous rocks, composed in great part of grey and greenish sandstone and conglomerate, with some thick beds of nummulitic limestone.

The conglomerate contains pebbles of quartz, limestone, and Lydian stone.

This system of rocks is very extensively spread throughout Catalonia; one of its members being a red sandstone, to which the celebrated salt-rock of Cardona, usually considered as of the cretaceous era, is subordinate.

Near Amer, in the Valley of the Ter, on the southern borders of the region delineated in the map, primary rocks are seen, consisting of gneiss, mica-schist, and clay-slate. They run in a line nearly parallel to the Pyrenees, and throw off the fossiliferous strata from their flanks, causing them to dip to the north and north-west. This dip, which is towards the Pyrenees, is connected with a distinct axis of elevation, and prevails through the whole area described in the map, the inclination of the beds being sometimes at an angle of between 40 and 50 degrees.

It is evident that the physical geography of the country has undergone no material change since the commencement of the era of the volcanic eruptions, except such as has resulted from the introduction of new hills of scoriae, and currents of lava upon the surface. If the lavas could be remelted and poured out again from their respective craters, they would descend the same valleys in which they are now seen, and re-occupy the s.p.a.ces which they at present fill. The only difference in the external configuration of the fresh lavas would consist in this, that they would nowhere be intersected by ravines, or exhibit marks of erosion by running water.

_Volcanic cones and lavas._--There are about fourteen distinct cones with craters in this part of Spain, besides several points whence lavas may have issued; all of them arranged along a narrow line running north and south, as will be seen in the map. The greatest number of perfect cones are in the immediate neighbourhood of Olot, some of which (Nos. 2, 3. and 5.) are represented in the annexed woodcut; and the level plain on which that town stands has clearly been produced by the flowing down of many lava-streams from those hills into the bottom of a valley, probably once of considerable depth, like those of the surrounding country.

[Ill.u.s.tration: Fig. 471. View of the Volcanos around Olot in Catalonia.]

In this drawing an attempt is made to represent, by the shading of the landscape, the different geological formations of which the country is composed.[410-A] The white line of mountains (No. 1.) in the distance is the Pyrenees, which are to the north of the spectator, and consist of hypogene and ancient fossiliferous rocks. In front of these are the fossiliferous formations (No. 4.) which are in shade. The hills 2, 3. 5.

are volcanic cones, and the rest of the ground on which the suns.h.i.+ne falls is strewed over with volcanic ashes and lava.

The Fluvia, which flows near the town of Olot, has cut to the depth of only 40 feet through the lavas of the plain before mentioned. The bed of the river is hard basalt; and at the bridge of Santa Madalena are seen two distinct lava-currents, one above the other, separated by a horizontal bed of scoriae 8 feet thick.

In one place, to the south of Olot, the even surface of the plain is broken by a mound of lava, called the ”Bosque de Tosca,” the upper part of which is scoriaceous, and covered with enormous heaps of fragments of basalt, more or less porous. Between the numerous hummocks thus formed are deep cavities, having the appearance of small craters. The whole precisely resembles some of the modern currents of Etna, or that of Come, near Clermont; the last of which, like the Bosque de Tosca, supports only a scanty vegetation.

Most of the Catalonian volcanos are as entire as those in the neighbourhood of Naples, or on the flanks of Etna. One of these, called Montsacopa (No.

3. fig. 471.), is of a very regular form, and has a circular depression or crater at the summit. It is chiefly made up of red scoriae, undistinguishable from that of the minor cones of Etna. The neighbouring hills of Olivet (No. 2.) and Garrinada (No. 5.) are of similar composition and shape. The largest crater of the whole district occurs farther to the east of Olot, and is called Santa Margarita. It is 455 feet deep, and about a mile in circ.u.mference. Like Astroni, near Naples, it is richly covered with wood, wherein game of various kinds abounds.

[Ill.u.s.tration: Fig. 472. Cross section.

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