298 J. W. JUDD ON THE ANCIENT VOLCANO OF 



sites of the surrounding volcanic girdle, and indeed pass into the 

 latter by the most insensible gradations ; hence by the more modern 

 writers on the district they have been called "greenstone-trachytes," 

 while Bichthofen suggested for them the name of " propylites," a 

 term which has not, however, met with any thing like general 

 acceptance among petrologists. 



The " greenstone-trachytes " of Hungary are rocks usually con- 

 sisting of a more or less felspathic base, in which large crystals of 

 plagioclase felspar with others of hornblende and mica are im- 

 bedded, so as to give them a more or less strikingly porphyritic 

 character. In places, however, the rock becomes granular, and 

 sometimes almost compact, in which cases it is dark-coloured and is 

 known locally as " aphanitic greenstone." Occasionally this rock 

 is found to contain spherical concretionary masses, and is then 

 known as " Kugelgriinstein ;" sometimes it contains irregular 

 cavities, which are lined with various crystallized minerals. Occa- 

 sionally the greenstone-trachytes contain grains of free quartz, 

 and so pass into " dacites " or quartziferous-andesites of porphy- 

 roid or granitoid structure. Such rocks present the closest analo- 

 gies with the quartz-diorites of the older geological periods. 



The greenstone-trachytes are clearly intrusive rocks, and are 

 found traversing the Triassic strata in dykes, and also enclosing in 

 their midst enormous masses of these latter deposits, which have 

 been subjected to intense metamorphism. Many portions of these 

 greenstone-trachytes have undergone great alteration, evidently 

 through having been traversed by acid vapours ; and they now con- 

 tain, disseminated through their mass, large quantities of pyrites 

 and other metallic minerals, including gold and silver. The rock 

 occasionally assumes a brecciated appearance and would appear at 

 times to constitute a true " friction-breccia ;" some of the masses 

 which have been most profoundly altered by the percolation through 

 them of acid gases have assumed a white and earthy appearance, 

 and have been called, though erroneously, " greenstone-tuffs." 

 The greenstone-trachytes are traversed by veins of quartz, often of 

 large size, which constitute the principal receptacles of those valu- 

 able metallic ores for which the district is so famous. These 

 metallic lodes traverse the so-called " syenite and granite," the 

 M greenstone-trachyte," and the various stratified rocks through 

 which these igneous masses have been extruded. The most pro- 

 ductive deposits of ore, however, appear to be found near the 

 junctions of the intrusive masses with the metamorphosed sedimen- 

 tary rocks. 



The beautiful so-called syenitic rock of the Schemnitz area forms 

 four distinct bosses, which are surrounded by highly metamor- 

 phosed rocks, and are at the same time very intimately associated 

 with the greenstone-trachytes. The rock in question is a per- 

 fectly crystalline aggregate, either coarse or fine-grained, and occa- 

 sionally porphyritic in structure. So strikingly granitic is its aspect, 

 that all geologists who have studied the district have named the 

 rock either syenite or granite, according to the absence or presence 



