310 J. W. JTTDD ON THE ANCIENT VOLCANO OP 



According to the analyses of Karl von Hauer, the felspar in the 

 different varieties of the Hungarian rocks would appear to be usually 

 either labradorite or some variety intermediate between that species 

 and andesite. The results obtained by Szabo by a totally different 

 mode of investigation are not very divergent from those arrived at by 

 Yon Hauer. 



The microscopic examination, which proves that the mineralo- 

 gical constituents of all these rock-varieties — the andesitic lavas, the 

 " greenstone-trachytes," and the " syenite and granite " of Hodritsch 

 — are quite identical, consisting in each case principally of a plagio- 

 clase felspar with orthoclase, hornblende, mica, and sometimes quartz 

 superadded, also demonstrates what are the real differences, namely 

 those of structure, which serve to distinguish, though not by any 

 hard lines of demarcation, the three several groups of rocks. The 

 materials of the so-called granite and syenite are all distinctly 

 crystallized; the "greenstone-trachytes" consist of a micro-crys- 

 talline or felsitic base, in which the crystals, often injured and 

 broken, are imbedded ; while in the andesite lavas the base is usually 

 of a more compact texture, passing into a glassy condition. I hope 

 to be able to show in the sequel that these variations of structure 

 are fully accounted for by the differences of the conditions under 

 which the several kinds of rocks can be shown to have become con- 

 solidated. 



Having pointed out what appear to me to be the true relations of 

 the several varieties of igneous rocks in this complicated district, I 

 will now proceed to notice the different explanations which have 

 been offered of its general physical structure. Beudant correctly 

 described the volcanic rocks of Hungary as constituting five well- 

 marked and more or less completely isolated mountain-groups — those, 

 namely, of (1) the Schemnitz Mountains, (2) the Yisegrad Moun- 

 tains, (3) the Matra, (4) the Tokay-Eperies group, (5) the Vihorlat 

 Mountains ; and to these we must add, for Transylvania, two other 

 groups (6) the Hargitta Mountains and (7) the Transylvanian 

 Erzgebirge. Other similar masses of volcanic rocks of con- 

 temporaneous date and analogous composition stretch through 

 the provinces of European Turkey, forming a link between the 

 Miocene volcanic rocks of our continent and those of Asia. So 

 enormously have the volcanic rock-masses of Hungary suffered 

 from denuding causes, and so much more difficult is it to trace the 

 relations between the isolated fragments of their short and bulky 

 trachytic lava-streams, as compared with the wide-spreading plateaux 

 constituted by the basaltic lavas of Auvergne, that it is not surpri- 

 sing to find Beudant altogether failing to perceive any connexion 

 between the different isolated masses, and declaring that almost 

 every separate mountain appears to be the result of a distinct 

 eruption. 



It is to Yon Pettko, whose studies and writings have thrown so 

 much new light upon the structure of the Schemnitz district, that 

 we are indebted for the first clear exposition of the true relations of 

 the different masses of volcanic rocks, and for some most valuable 



