SZABO TOKAY. 15 



contact with sea-water, which, in the course of time, alters it into 

 vitreous clear- coloured varieties, then into pumice, and finally into a 

 white vitreous powder, which, mixed with larger fragments of theso 

 and the preceding products of alteration, constitutes a pumice-con- 

 glomerate. The vitreous varieties give rise to obsidian- and perlite- 

 porphyries. 



h. Lithoid rhy elites, the substance of the upper portion of a rhyolitic 

 volcano, seem to have been produced by a later eruption, beginning 

 with vitreous substances, and pass'ng upwards into genuine lithoid 

 lavas. Dark colours are of rare occurrence among them; the in- 

 cluded particles of quartz and felspar diminish to a very small size, 

 and sometimes the substance appears homogeneous. The lower por- 

 tions are homogeneous obsidian, the result of rapid solidification ; or 

 genuine perhte, sometimes with a nucleus of obsidian. Sphaerulitcs 

 of radiated or dendritic structure are segregated from the rhyolitcs 

 of the Hagyallya. 



c. Sedimentary breccias and tuft's, as mere agglomerations near 

 the places of eruption, and at some distance from them more or 

 less stratified with marine shells, as Cerithium lignitarum, Area, and 

 Pecten. These strata, solidified by infiltration, are Beudant's " mill- 

 stone porphyries." The younger tufaceous strata, subsequently 

 solidified by access of silica, lie nearly horizontallj^ and contain 

 species of Cerithium and Cardimn of the Cerithian strata. 



d. Ehyolitic trass, resting on the primary volcanic tuff's, eff'erves- 

 cing with acids, and partly decomposed into clay. The non-volatile 

 substances contained in the water of the sea, beneath whose level 

 these rhyolites have been raised, have had a chemical action on them. 

 In the first place, the oxide of iron has been converted into a soluble 

 substance, sesquichloride of iron, which, having been washed out, 

 left rocks of gradually paler tints, until the uppermost of them 

 became completely white. Then an addition of sodium may have 

 made them more fusible ; and the sulphur of the pyrites which 

 was originally included in them, and of which traces still occur, 

 may have been oxidized into sulphuric acid, still occurring in the 

 white varieties of rhyolite (probably in the form of a mechanical 

 aggregation of sulphates and silicates). When submitted to red heat 

 and to decomposition under the action of water, these white rocks 

 treated with hot water give a considerable proportion of alum. An 

 extensive upheaval took place at the end of the rhyolitic period ; and 

 in consequence of it, several closed freshwater basins, excavated in 

 rhyolitic breccias and tuff's and receiving the waters of siliciferous 

 thermal springs, were formed around the margin of the group of 

 rhyolitic islands. The lowermost deposit in these basins is a stratum 

 of silicified mud, with specimens of Planorhis deprived of their calca- 

 reous shell. Then follows quartzite, with plenty of silicified trunks 

 of trees and other vegetable fragments, all transported by waters 

 from without, — or menilite, easily decomposed by water, the soluble 

 variety of silica occurring in the upper strata under the form of 

 white, loose, and very thin layers composed of Diatomacece, with 

 fine impressions of leaves on their planes of contact. 



