THE DISTRICT OP BCHEMNITZ, HUNGAfcr. 3^1 



the " syenite." Of the former it certainly represents the last stage 

 of alteration ; but with the latter it has absolutely nothing in com- 

 mon. It is a rock of far more acid composition, and it contains 

 much free quartz and orthoclase felspar ; while the igneous masses in 

 contact with it have little or no free quartz, and the felspar is prin- 

 cipally plagioclase. 



The phenomena presented by these masses of aplite can only be 

 explained, as it appears to me, by regarding the rock as the last 

 stage of metamorphism of the Triassic sediments. That their par- 

 ticles must have attained the condition of almost absolute internal 

 mobility is shown by the nearly perfectly crystalline character of 

 the rock ; and that, as a consequence of this, the mass was in a 

 truly plastic state is shown by the fact that it was capable of being 

 injected in the form of veins into the fissures of the surrounding 

 masses. To the same cause (its perfect internal mobility), as I 

 shall hereafter show, we mast assign that disappearance of the folia- 

 tion which distinguishes the surrounding gneissic rocks, into which 

 it frequently passes, however, by the most insensible gradations. 



We thus see that the examination of the ruins of the old Hun- 

 garian volcano leads us to results identical with those which we 

 obtain by the study of the similar ones of the same age in the West- 

 ern Isles of Scotland — namely, that portions of the same liquefied 

 masses of rocks, which at the surface flowed in lava-streams or 

 were scattered by explosions as scoria) or ashes, consolidated at a 

 great depth and under considerable pressure in the interior of the 

 volcano, in a highly crystalline and even perfectly granitic condi- 

 tion. AYe have also shown that around these vast and slowly 

 consolidating masses of intensely heated rock, a series of chemical 

 changes were set up in the sediments which they penetrated, 

 resulting in the transformation of the latter into schistose and 

 highly crystalline rocks. 



As in the case of the rock-masses of intermediate composition 

 (the andesites and diorites) we find the more deep-seated portions 

 assuming a highly crystalline and even granitic character, so we 

 also have grounds for the conclusion that, under the same condi- 

 tions, the rhyolitic and vitreous rocks of acid composition pass 

 into highly crystalline quartz-trachytes, presenting precisely those 

 characters which are regarded by some penologists as belonging 

 only to rocks of very ancient date. This highly porphyntic or semi- 

 granitic form of the rhyolitic rocks is well exhibited by the great 

 intrusive masses near Eisenbach and Konigsberg. 



Of the interesting minerals which result from the long-continued 

 reactions taking place between masses of highly heated volcanic 

 materials and the surrounding sedimentary rocks — and with which 

 we are so familiar at Monzoni, an old volcanic vent in which they 

 are seen lying in their original situation, and at Vesuvius, where 

 they are continually being ejected from the crater — we find several 

 very interesting examples in the case of the great Schemnitz vol- 

 cano. Thus, near Hodritsch, a mass of Trias dolomite entangled 

 in the dioritic rock exhibits, near the planes of its junction with the 



