has been in contact with the atmosphere, and where refrigeration has been most rapid, is always found to consist of scoriform, vitreous, and porous lava; while at the greater depth the mass assumes a more lithoidal structure, and then becomes more and more stony as we descend, until at length we are able to recognize with a magnifying glass the simple minerals of which the rock is composed. On penetrating still deeper, we can detect the constituent parts by the naked eye, and in the Vesuvian currents distinct crystals of augite and leucite become apparent.

The same phenomenon, observes M. Necker, may readily be exhibited on a smaller scale, if we detach a piece of liquid lava from a moving current. The fragment cools instantly, and we find the surface covered with a vitreous coat; while the interior, although extremely fine-grained, has a more stony appearance.

It must, however, be observed, that although the lateral portions of the dikes are finer grained than the central, yet the vitreous parting layer before alluded to is rare in Vesuvius. This may, perhaps, be accounted for, as the above-mentioned author suggests, by the great heat which the walls of a fissure may acquire before the fluid mass begins to consolidate, in which case the lava, even at the sides, would cool very slowly. Some fissures, also, may be filled from above, as frequently happens in the volcanoes of the Sandwich Islands, according to the observations of Mr. Dana; and in this case the refrigeration at the sides would be more rapid than when the melted matter flowed upwards from the volcanic foci, in an intensely heated state. Mr. Darwin informs me that in St. Helena almost every dike has a vitreous selvage.

The rock composing the dikes both in the modern and ancient part of Vesuvius is far more compact than that of ordinary lava, for the pressure of a column of melted matter in a fissure greatly exceeds that in an ordinary stream of lava; and pressure checks the expansion of those gases which give rise to vesicles in lava.

There is a tendency in almost all the Vesuvian dikes to divide into horizontal prisms, a phenomenon in accordance with the formation of vertical columns in horizontal beds of lava; for in both cases the divisions which give rise to the prismatic structure are at right angles to the cooling surfaces. (See above, p. 617.)

CHAPTER XXXI.

ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS, continued.

Volcanic rocks of the Newer Pliocene period-Val di Noto-Sicilian dikes-Region of Olot in Catalonia-Volcanic rocks of the Older Pliocene period-TuscanyRome-Volcanic region of Olot in Catalonia-Cones and lava-currents-Ravines and ancient gravel-beds-Jets of air called Bufadors-Age of the Catalonian volcanoes-Upper Miocene period-Volcanic archipelagoes of Madeira, the Canaries, and the Azores-Lower Miocene period-Brown-coal of the Eifel and contemporaneous trachytic breccias-Age of the brown-coal-Peculiar characters of the volcanoes of the upper and lower Eifel-Lake Craters-Trass-Hungarian volcanoes.

VOLCANIC ROCKS OF THE NEWER PLIOCENE PERIOD.

Val di Noto.-I have already alluded (see p. 192) to the igneous rocks which are associated with a great marine formation of limestone, sand, and marl in the southern part of Sicily, as at Vizzini and other places. In this formation, which was shown to belong to the Newer Pliocene period, large beds of oysters and corals repose upon lava, and are unaltered at the point of contact. In other places we find dikes of igneous rock intersecting the fossiliferous beds, and converting the clays into siliceous schist, the laminæ being contorted and shivered into innumerable fragments at the junction, as near the town of Vizzini.

The volcanic formations of the Val di Noto usually consist of the most ordinary variety of basalt, with or without olivine. The rock is sometimes compact, often very vesicular. The vesicles are occasionally empty, both in dikes and currents, and are in some localities filled with calcareous spar, arragonite, and zeolites. The structure is, in some places, spheroidal; in others, though rarely, columnar. I found dikes of amygdaloid, wacke, and prismatic basalt, intersecting the limestone at the bottom of the hollow called Gozzo degli Martiri, below Melilli.

Dikes in Sicily.-Dikes of vesicular and amygdaloidal lava are also seen traversing marine tuff or peperino, west of Palagonia, some of the pores of the lava being empty, while others are filled with carbonate of lime. In such cases we may suppose the peperino to have resulted from showers of volcanic sand and scoriæ, together with fragments of limestone, thrown out by a submarine explosion, similar to that which gave rise to Graham Island in 1831. When the mass

was, to a certain degree, consolidated, it may have been rent open, so that the lava ascended through fissures, the walls of which were perfectly even and parallel. After the melted matter that filled the rent (fig. 716) had cooled down, it must have been fractured and shifted horizontally by a lateral movement.

In the second figure (fig. 717) the lava has more the appearance of

b. Peperino, consisting of volcanic sand, mixed with fragments of
lava and limestone.

a vein which forced its way through the peperino. It is highly probable that similar appearances would be seen, if we could examine the floor of the sea in that part of the Mediterranean where the waves have recently washed away the new volcanic island; for when a superincumbent mass of ejected fragments has been removed by denudation, we may expect to see sections of dikes traversing tuff, or, in other words, sections of the channels of communication by which the subterranean lavas reached the surface.

Volcanic Rocks of Olot in Catalonia.-Geologists are far from being able, as yet, to assign 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 in part to the Post-pliocene and in part to the Newer Pliocene period, a district of extinct volcanoes 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.

Dr. Maclure, the American geologist, was the first who made known the existence of these volcanoes; and, according to his de

*Maclure, Journ. de Phys., vol. Ixvi. p. 219, 1808; cited by Daubeny, Descrip tion of Volcanoes.

scription, the volcanic region extended over twenty square leagues, from Amer to Massanet. I searched in vain in the environs of Massanet 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 gray 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, crystalline 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 northwest. 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 scoriæ, 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 reoccupy the spaces 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. greatest number of perfect cones are in the immediate neighborhood of Olot, some of which (fig. 719, Nos. 2, 3, and 5) are represented in

Fig. 719.

The

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.

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.* The white line of mountains (No. 1) in the

This view is taken from a sketch which I made on the spot in 1830.