300 J. W. JTTDD ON THE ANCIENT VOLCANO OF 



found assuming the perfectly glassy character and becoming true 

 obsidians, like those of Lipari, Iceland, Mexico, Ascension, &c; 

 their lustre scarcely ever passes beyond the semivitreous or resi- 

 nous ; and hence they may be rather classed with the pitchstones, 

 or, being frequently porphyritic in structure, with the " pitchstone- 

 porphyries." Throughout Hungary, indeed, true obsidians are very 

 rare ; and although most interesting varieties of this rock of black, 

 red, and green colours are found in the Tokay-Hegyalja, some of 

 which have been well described by Vogelsang, yet, so far as my own 

 observations go, these never form large rock-masses as in Lipari, but 

 occur only as " Saalbands " on the sides of intrusive dykes of rhyo- 

 litic or quartz-trachyte rock. 



Of the different varieties of the curious perlitic variety of structure, 

 however, the Hungarian rhyolites, and especially those of the vicinity 

 of Schemnitz, afford the most interesting and instructive examples. 

 Indeed almost all the vitreous rocks of Hungary exhibit, under the 

 microscope, the presence of those peculiar and little-understood 

 spiral and concentric structures so characteristic of perlite ; and in 

 addition to these we frequently find various modifications of true 

 sphrerulitic concretions which are always quite independent of, and 

 evidently owe their formation to quite different causes from the 

 former. 



Although the stony and slightly vitreous varieties of the Schem- 

 nitz rhyolites frequently assume a scoriaceous character, and thus 

 give rise to the formation of those beautiful " millstone- porphyries" 

 which are so extensively worked at Do Sminje, near Hlinik, and at 

 Himmelreich, near Konigsberg, yet pumice is rare. The so-called 

 pumice of the Hlinikcr Thai presents, indeed, a slight approximation 

 to the typical characters of this rock, and, under the microscope, is 

 seen to be full of gas-pores ; yet the sponge-like character which in 

 true pumice is so manifest to the naked eye, is almost always want- 

 ing in the Hungarian rock, and I have never found specimens of the 

 latter that would float in water. This may in part be accounted 

 for, no doubt, by the circumstance that the rhyolitic rocks have un- 

 dergone a very considerable amount of denudation, by which the 

 lighter and more fragile materials would be easily swept away; but 

 I believe there is also evidence that a considerable amount of subsi- 

 dence took place before the extrusion of these rhyolitic lavas, and 

 that they were erupted under a pressure of water sufficient to pre- 

 vent the full distention of the liquefied mass by the escaping gases 

 and vapours. 



Through the so-called hornstone modification, the semivitreous 

 and perlite rocks of Hungary graduate into the stony varieties. 

 These exhibit great diversities of character, being sometimes exceed- 

 ingly compact and fine-grained, and at others highly porphyritic, in 

 the latter case frequently exhibiting approximations towards the 

 granitic structure. The beautiful red rhyolite of the " Stein-Meer," 

 near Eisenbach, containing large but imperfect double pyramids and 

 rounded grains of quartz, is almost identical in appearance and 

 characters with many quartz -porphyries of the older geological • 

 periods. 



