34 GEOLOGICAL MEMOIRS. 



" trachytic porphyry." Certain accidental modifications of structure 

 received the denomination of perlite, pumice, obsidian, &c, together 

 with a special place in the system. The geological facts within the 

 Hungarian mountains and our present knowledge of the phenomena 

 of solidification dissipate any doubt concerning the narrow relation 

 between these varieties, as already supposed by Beudant. The 

 general denomination of " trachytic porphyry " would imply some- 

 thing quite different from its real signification and be a source 

 of many errors. Rhyolite, as the denomination of a whole group, 

 would at least draw attention to a common character of all its sub- 

 divisions, all bearing traces of igneous fusion, and resembling either 

 glass or China-ware, or real lava-currents. 



The series in geological age, beginning with the most ancient 

 group, is — greenstone, trachyte, rhyolites, and basalts. The first 

 two are everywhere in essential mutual connexion. Basalt is inde- 

 pendent of both, and appears in isolated groups, rarely encroaching 

 on their dominions. Trachyte has exclusively broken out to day in 

 masses following extensive and precisely determined fissures, and 

 towering into large mountain-ranges. Bhyolite, as it were a para- 

 sitical formation, occupies the flanks or the basis of the trachytic 

 groups or ranges, appearing but rarely and very subordinately in 

 massive eruptions. It is essentially a result of genuine volcanic 

 activity, an ancient lava, having flowed out of craters, or through 

 fissures in the walls of volcanos or in the slopes of t&achytic ranges. 

 The basalt is equally eruptive and volcanic. The eruption of green- 

 stone-trachyte is a continental one. Subsequently to them the level 

 of the land was lowered, so that the grey trachytes were enveloped 

 at the very moment of their breaking out by layers of tuffs, partly 

 alternating with them. 



These trachytic tuffs are of high importance in the tertiary erup- 

 tive formations of Hungary. The series of craters lying along the 

 foot of the trachytic mountains, which gave issue to the rhyolitic 

 current, began to be active only after the completion of the trachytic 

 eruption and the subsequent invasion of sea upon the dry land. 

 These eruptions bear over a large area well-defined marks of peri- 

 odical modifications in the nature and mode of formation of the 

 eruptive rocks, testifying to a gradual emergence of land and retreat 

 of sea during the rhyolitic period. It begins with submarine erup- 

 tions, and ends with a series of diminutive eruptions on dry land. 

 Another depression may have taken place previously to the eruption 

 of basalts, these being likewise connected with considerable deposits 

 of tuff. 



The three groups of Tertiary eruptive rocks, as proposed here, are 

 not confined to the South Carpathian slopes. We find them again in 

 Central Germany, where the first two of them are almost wanting ; 

 in Asia Minor and on the Armenian plateau ; in the Euganeans, where 

 the trachytic group, as the most ancient, is followed by the perlites 

 of the rhyolitic group, and this, apart from the rest, by the basalts 

 of the Vicentin ; in Iceland, where the basic compounds have taken 

 the place of the rhyolites ; in New Zealand (according to Dr. Hoch- 



