376 Notices of Memoirs — Classification of the EriqAice Rocks. 



pumiceons, and assumes an appearance wliicli Beudant lias desig-nated 

 semi-vitreous. It is also often possible to follow tlie transition from 

 the normal to the rhyolitic condition ; the amphibole and augite 

 being the first to disappear, since they are the minerals which fuse 

 most easily. 



Of all the felspars, orthose is the most remarkable for possessing 

 the power of undergoing hydration. This it does more easily than 

 the soda and lime felspars ; so that, in the obsidians, pitchstones, 

 and porphyritic perlites, where we find quartz, mica, and felspar, the 

 last is alwaj's oligoclase, while the potash is met with in the vitreous 

 mass. 



Lithoidite is the product of devitrification of the rh3'olites. 

 Trachytic-greensfone has a different composition, and varieties of it 

 occur, just as the normal trachytes vary. It is possible to follow 

 very clearly the natural transition from the normal condition of 

 trachyte to that of greenstone. This transition results from sub- 

 sequent solfataric action. The sulphurous and metallic emanations 

 have impregnated a certain region of trachyte of one or another 

 type, producing in it a series of changes which are still going on. 

 The matter resulting from these changes has given rise to the 

 formation of metalliferous lodes. The name trachytic-greenstone is 

 useful and even necessary for the miner, but, for the geologist, it has 

 no existence as a special formation ; a jpropylitic eruption has never 

 taken place. 



Domite is a modification of an old type, caused by the volcanic 

 action of a later eruption, especially by the emanations of hydro- 

 chloric acid which have removed the iron from the magnetite and 

 the ferruginous minerals, but which has not altered the felspar, 

 which in domite is always glassy. 



Millstone-porphyry is a siliceous modification of an earlier trachytic 

 type, which occurs either in a massive condition, or in the form of 

 breccias or conglomerates. The millstone-porphyry of Sarospatak 

 of Beudant, containing well-preserved sanidine and doubly-terminated 

 pyramids of quartz, is simply breccia containing Middle Miocene 

 fossils. 



Almiite is the modification caused by emanations containing 

 sulphuric acid, which decomposes the felspathic silicates. One 

 always finds in the alunites masses of quartz, millstone-quartz 

 (Silex-meuliere), and if there has been a persistent disengagement 

 of sulphuric acid, the vapour of water has carried off the alkaline 

 sulphates, and the sulphuric acid has combined with the alumina. 

 In this way the kaolin, which always accompanies alunite, has been 

 formed. I have found silicified wood in the alunite deposits of 

 Beregszaz, and the remains of Upper Miocene Molluscs at Sarospatak. 



Kegarding the trachytic formation as a whole, it should be con- 

 sidered as an eruptive unit, and may be termed a cycle of eruption. 

 I am convinced that the trachytes of Hungary, those of Servia, 

 which are but a continuation of them, as well as those of the 

 Enganeen Hills in Italy, belong to the same cycle. They are con- 

 temporaneous, and correspond with one another. This probably 



