THE DISTRICT OF SCHEMNITZ, HUNGARY. 299 



in it of free quartz. Microscopic study of this rock, however, 

 shows that its predominating constituent is always a plagioclase 

 felspar, to which hornblende and mica, and a variable quantity of 

 orthoclase is added. This rock ought therefore, it is clear, to be 

 classed with the diorites rather than with the syenites ; but its 

 felspar is probably not oligoclase but labradorite. When, as is 

 frequently the case, a considerable quantity of free quartz is present 

 in it, the rock, though called a " granite," should be rather referred 

 to the quartziferous diorites. The rook with which these granitic 

 masses of Hungary appear to have the closest points of resemblance 

 is the so-called granite of the Adamello group in the Alps, which 

 Vom liath has distinguished under the name of Tonalite. Of the 

 exact relations of these bosses of granitic rock in the Schemnitz 

 district to the stratified, metamorphic, and volcanic rocks which 

 surround them we shall have to treat more fully in the sequel. 



Besides those volcanic rocks of the Schemnitz area already de- 

 scribed, and which belong, as we have seen, to the class intermediate 

 in composition between the acid and basic types, there are evidences 

 of a number of eruptions of rocks of totally different composition, 

 almost all of which are of later date than the widely spread ande- 

 sitic lavas of Hungary. They belong either to the extremely acid 

 class (of liparites, perlites, obsidians, &c.) or to the extremely basic 

 type (basalts and augite-andesites). 



Of certainly later date than the great masses of amphibole-ande- 

 sites, with their associated " greenstone- trachytes " and dacites 

 (propylites), are those interesting rhyolitic rocks which rise in the 

 very centre of the district we are describing. They are espe- 

 cially well exposed at Konigsberg, near Vichnye (Eisenbach), in 

 the Hliniker Thai, and in the country north of the Gran, extending 

 from Heiligen Kreuz, nearly as far as Kremnitz (see the map and 

 section, Plate XX.). These rocks present, in their chemical com- 

 position, in their mineralogical constitution, and in their general 

 features, the most marked contrasts with the andesitic rocks which 

 cover so large a portion of the surrounding country. The Hungarian 

 rhyolites contain from 70 to 81 per cent, of silica ; the proportion 

 of iron and of the alkaline earths is very small in them, while that 

 of the alkalios is very high, the quantity of the potash usually ex- 

 ceeding that of the soda. These rhyolites always contain free 

 quartz, sometimes in considerable quantity; the predominating 

 felspar in them is always orthoclase; and their other principal 

 ingredients are hornblende and biotite, which occur sometimes 

 separately, and at others in association with one another. 



The features of greatest interest presented by these rhyolitic 

 rocks, however, are those connected with their structure. They ex- 

 hibit, as is so constantly the case with rocks of the most acid class, 

 a great tendency to assume the vitreous character ; and hence arise 

 all those interesting modifications of rock-structure which were 

 first clearly described by Beudant in 1822, and which have made 

 the locality of Hlinik so famous among petrologists. 



In the Schemnitz district, however, the rhyolitic rocks are seldom 

 a J. G. S. No. 127. t 



