544 HUNGAEY. [Ch. XXXI. 



rous trunks and branches of trees. If tliis trass was formed during the 

 period of volcanic eruptions it may perhaps have originated in the man- 

 ner of the moya of the Andes. 



We may easily conceive that a similar mass might now be produced, 

 if a copious evolution of gases should occur in one of the lake basins. 

 The water might remain for weeks in a state of violent ebullition, until 

 it became of the consistency of mud, just as the sea continued to be 

 charged with red mud round Graham's Island, in the Mediterranean, in 

 the year 1831. If a breach should then be made in the side of the 

 cone, the flood would sweep away great heaps of ejected fragments of 

 shale and sandstone, which would be borne down into the adjoining 

 valleys. Forests might be torn up by such a flood, and thus the occur- 

 rence of the numerous trunks of trees dispersed irregularly through the 

 trass, can be explained. 



Hungary. — M. Beudant, in his elaborate work on Hungary, describes 

 five distinct groups of volcanic rocks, which, although nowhere of great 

 extent, form striking features in the physical geography of that country, 

 rising as they do abruptly from extensive plains composed of tertiary 

 strata. They may have constituted islands in the ancient sea, as Santo- 

 rin and iMilo now do in the Grecian Archipelago; and M. Beudant has 

 remarked that the mineral products of the last-mentioned islands resem- 

 ble remarkably those of the Hungarian extinct volcanos, where many 

 of the same minerals, as opal, chalcedony, resinous silex (silex resinite)^ 

 pearlite, obsidian, and pitchstone abound. 



The Hungarian lavas are chiefly felspathic, consisting of different 

 varieties of trachyte ; many are cellular, and used as millstones ; some 

 so porous and even scoriform as to resemble those which have issued in 

 the open air. Pumice occurs in great quantity ; and there are conglom- 

 erates, or rather breccias, wherein fragments of trachyte are bound 

 together by pumiceous tuff, or sometimes by silex. 



It is probable that these rocks were permeated by the waters of hot 

 springs, impregnated, like the Geysers, with silica ; or in some instances, 

 perhaps, by aqueous vapours, which, like those of Lancerote, may have 

 precipitated hydrate of silica. 



By the influence of such springs or vapours the trunks and branches 

 of trees washed down during floods, and buried in tuffs on the flanks 

 of the mountains, are supposed to have become silicificd. It is scarcely 

 possible, says M. Beudant, to dig into any of the pumiceous deposits of 

 these mountains without meeting with opalized wood, and sometimes 

 entire silicified trunks of trees of great size and weight. 



It appears from the species of shells collected principally by M. Boue, 

 and examined by M. Deshayes, that the fossil remains imbedded in the 

 volcanic tuffs, and in strata alternating with them in Hungary, are of 

 the Miocene type, and not identical, as was formerly supposed, with the 

 fossils of the Paris basin. 



