JOKELY GEOLOGY OF NORTH-WESTERN BOHEMIA. 15 



tions, the porphyries are contemporaneous with the Werfen-strata. 

 The tuffs are either tufaceous sandstones intimately connected with 

 the porphyries themselves, and therefore contemporaneous with 

 them, or of an aspect resembling silex, and evidently Tertiary beds 

 metamorphosed by the contact, or at least the proximity, of the por- 

 phyries, in a way still unexplained. Near Tiiffer the Tertiaiy marl- 

 slates next to the porphyries are abruptly metamorphosed into 

 corneous silex or tufaceous sandstone. [Count M.] 



On Hie Geology of North-western Bohemia. By M. Jok£ly. 



[Proceed. Imp. Geol. Instit. Vienna, January 25, 1859.] 



The mass of the Iser-Gebirge, representing the central nucleus of 

 the Sudetian mountains, is granitite, a compound of oligoclase and 

 interspersed binary crystals of constantly pale-reddish orthoclase. 

 Granite is but of secondary occurrence, either as fragments impasted 

 in granitite, or distributed in irregvdar masses among the gneiss sur- 

 rounding the southern slope of the granitite, or rising as so many 

 islands above the diluvial deposits. 



The Jescken Mountain-group joins the Iser-Gebirge on the south- 

 east. Its chief constituents are argillaceous slate, with local beds 

 of quartzite, granular limestone, amphibolic rocks, and greywacke- 

 like slates with intercalated gneiss, probably of eruptive origin. 

 Manifold irregularities in the stratification of these rocks may be 

 ascribed to a pressure which has acted on them in the direction from 

 N. to S. 



Granite, with two distinct species of felspar, mica, and a dichroi't 

 quartz (Prof. Cotta's " Bumburg-granite ") is the essential consti- 

 tuent rock of the Bumburg-Hainspach Mountain-group. Another 

 variety of granite, with pale-reddish felspar, resembling imperfect 

 granite, forms local masses within the prevailing variety. Grey- 

 wacke and gneiss, with veins of galena and pyrites, like those of the 

 Jescken, and here and there amphibolic slates, are locally included 

 in the granite, being probably large fragments torn away from the 

 Jescken schistaceous masses, and essentially different from the veins 

 of massive diorite-likc amphibolites running locally through the 

 granitic rocks. 



From the circumstance of crystalline slates and gneiss everywhere 

 conformably overlying the granitite, this rock may well be supposed 

 to be the efficient cause of the last upheaval in the Sudetian moun- 

 tains. Granite, far less diffused, and having evidently taken but a 

 very secondary share in this convulsion, is undoubtedly of more remote 

 date than the granulite. The steep upheaval of the Old Red of 

 Licbenau and of the Cretaceous deposits (•• Ouadrr ") along the border 

 of the Jescken, and further off, may be of still less remote date, 

 being confined to a very limited space ; rolled fragments of granite 

 moreover occur in the Old Bed conglomi rate. Lying between mela- 

 phyre and porphyry. [Count M.] 



