Structure of the Eastern Alps. 399 



sea. It is much fissured, and the interstices are coated with crystals of carbo- 

 nate of hme, and with that variety of the mineral which is sometimes called 

 rock-milk. The building-stone is covered at the top of the hill with finely 

 laminated, micaceous, earthy beds ; very much, we believe, resembling the 

 sediments which, in the present day, are occasionally produced, on the shores 

 of the sea, during periods of volcanic eruption. 



The phenomena at Ferish and Straden occur on the western edge of the 

 volcanic region. To the east of these places, the tertiary sands are overlaid, 

 or broken through, by ridges of unmixed volcanic rocks. There are, we be- 

 lieve, no traces left of the ancient craters : but from the great predominance 

 of igneous rocks at Gleichenberg, that place may, perhaps, be considered not 

 far from the ancient centre of igneous eruption. The peaks which there rise 

 to the height of eight or nine hundred feet above the surrounding country 

 have all a trachytic character. In the ravines below the castle of Gleichen- 

 berg some of the subordinate masses are quarried for millstones; and are, appa- 

 rently, of a structure quite analogous to the millstone porphyry of Hungary, 

 described by M. Beudant. 



The base of this trachytic porphyry is chiefly composed of felspar, with much 

 disseminated black mica; and through it are distributed crystals of felspar, some 

 of which are in a very advanced state of decomposition : its prevailing colours 

 are reddish and greenish grey, with many irregular ferruginous stains. 



Between Gleichenberg and Hainfeld, the volcanic peaks diminish in height ; 

 and masses, which are within a short distance of each other and seem to form 

 but one group, change their character completely, from a coarse trachyte into 

 a cellular lava, from which they graduate into a basaltic lava, v»^ith a tendency 

 to globular structure. Some of these varieties, not however including any true 

 trachyte, occur in the woody cone of Steinberg south of the castle of Hainfeld, 

 and are seen to overlie fine-grained volcanic silt and conglomerate, which 

 repose on the yellow sands that slope away into the valley of the Raab*. We 

 also observed near the basaltic lava at the top of this hill, many fragments of 

 light, cellular scoria. 



From Hainfeld towards the north, the tertiary system of yellowish, micaceous 

 sand, marl, and uncemented beds of small pebbles, is uninterrupted for many 

 miles, forming a succession of round, woody hills : but at Riegisburg a mag- 

 nificent tabular mass of volcanic breccia, crowned by an ancient castle and 

 fortress, appears to cap the tertiary sands. This rock contains quartz pebbles 

 and much detritus of the tertiary groups, mixed up with scoria, and fragments 



* Plate XXXVI. fie. 15. 



