INTERIOR LAND CHANGES. 427 



take place from the summit as every eruption from the flanks does not produce a cone 

 and as great intervals of rest occur between successive explosions, in some instances 

 amounting to a century, the mere superficies of Etna develops a series of changes which 

 carry us back to a vastly remote date. 



However limited the field of active volcanic operation, and local the effects of the enor- 

 mous fires that occasionally flare up, and perpetually smoke, large tracts of country 

 furnish unquestionable proof of having formerly been scenes where " fire ran along upon 

 the ground," though no record exists of such explosions but that written by the eruptive 

 force upon the face of the territory subject to its ravages. The Roman plain is one of 

 these districts. Nowhere east of the Appenines, or in the central range, is any trace 

 discoverable of volcanic eruptions, except along a line drawn eastwards from Campania 

 to Horace's Mount Vultur a conical hill of lava and tuff, from whose sides issue car- 

 bonic acid and sulphuretted hydrogen. But a great part of the Campagna of Rome is, in 

 one sense or another, a volcanic formation, a fact which illustrates some of the traditions 

 of the Eternal City. Westphal and Hoffmann have minutely examined the geology of 

 the Roman plain, from whose memoirs upon the subject the following account of the 

 changes that have transpired is condensed. At a time when the sea washed the sides of 

 the Sabine and Volscian mountains, and the plain lay deep beneath the waters of the 

 ocean, it was thrown into disorder by the breaking out of numerous volcanoes. One of 

 the principal centres of disturbance was at the northern extremity in the Ciminian Hills, 

 where a chain of volcanic heights now appear, among which Monte Soriano has an eleva- 

 tion of 4000 feet above the level of the sea, and is covered with currents of trachytic 

 lava. The Alban Mount was another centre in the southern quarter, from which a great 

 stream of lava may be traced for rather more than eight miles, ending near the tomb of 

 Cecilia Metella, and was largely used by the old Romans as material for paving their 

 roads. Both centres discharged a prodigious quantity of ashes and scoria?, which by long 

 deposition beneath the waters became agglutinated into the earthy rock called tuff, of 

 which several varieties are frequent in large masses about Rome. The catacombs are 

 excavated in the " tufa granulare," a soft, dark-coloured, granular sort, which furnishes 

 the Roman cement of commerce. In the "tufa lithoides" a more compact rock the 

 cavities were quarried, which were afterwards the prisons of the Capitol. The Seven 

 Hills are composed of these volcanic products, resting upon marine alluvial deposits. In 

 that of the Capitol, upon a bed of sand and clay the uppermost marine formation- 

 there is granular tufa ten feet in thickness, and about a hundred feet of lithoid tuff over 

 it, rising to the summit. The Palatine, Viminal, Quirinal, and Pincian hills are chiefly 

 composed of granular tuff, which appears mixed with the lithoid in the Esquiline, 

 Caelian, and Aventine. For a long period after the volcanoes began to play, the sea must 

 have rested upon the plain, to allow of the formation of these tufuceous beds beneath its 

 waters, derived from the ashes and scoria?, and now overlying the alluvial marine strata* 

 At length the fires died away ; the waves withdrew, through the land rising suddenly or 

 by degrees ; a series of lakes remained, gradually drained by the Tiber and the Anio as 

 they scooped out their channels ; the Latins came down from the Appenines ; arid Rome 

 arose to become for a season the mistress of the Avorld. Of these physical changes we 

 have an enduring memorial graven, as with a pen of iron, upon the Campagna. Its lava 

 currents, its tufaceous masses, and its crateriform mountains, proclaim the occurrence 

 here of unwritten catastrophes. 



Equally significant are the appearances presented by a district in the southern pari of 

 Central France, more than forty miles in length and twenty in breadth, which was foi^ 

 merly comprised in the provinces of Auvergne and Languedoc. Here are found unques- 

 tionable evidences of long-extinguished subterranean fires, in more than two hundred 



