322 J. W. JUDD ON THE ANCIENT VOLCANO OF 



igneous mass, beautiful examples of ophicalcite ; and in a similar 

 situation at no great distance, masses of finely crystallized 

 fassaite and pleonaste also make their appearance. Epidote, mica, 

 garnet, and other minerals* are also found under similar conditions 

 at several points in the district. 



Although there are, as we have seen, so man}- points of resemblance 

 between the volcanoes of Hungary and those of Scotland, yet there is 

 one respect in which they afford a very remarkable contrast. In the 

 case of the Scottish volcanoes we observe but little evidence of those hot 

 and mineral springs and of those discharges of gas and vapour which 

 usually constitute the final stage of volcanic activity; while of the 

 existence and action of these, as we have seen, there are in the case 

 of the Hungarian volcanoes most abundant proofs. Now when we 

 come to examine the deep-seated rocks of each of these districts, 

 we find also a corresponding difference. While the rocks which once 

 formed the central masses of the old Scottish volcanoes have undergone 

 comparatively little chemical change since their eruption, those in 

 the midst of the Hungarian volcanoes have clearly been affected 

 in the most powerful manner through the penetration of their mass 

 by acid gases and vapours. Often the whole mass of rock over a 

 considerable area is found to be completely decomposed and reduced 

 to a white or variegated friable product, to which the name of 

 " greenstone-tuff" has been sometimes, though erroneously, applied. 

 At other times the change has been a more partial one ; and over 

 very considerable tracts the " greenstone-trachytes "or " propy- 

 lites " are found to be charged with the sulphides of iron, copper, 

 lead, and antimony, often containing some silver and gold. Where, 

 however, fissures have existed in these rocks, these various metallic 

 substances have been accumulated in greater abundance, and have 

 given rise to the formation of those valuable lodes for which the 

 district is so famous. 



It does not of course fall within the scope of the present memoir 

 to attempt to give any account of the great mineral deposits of the 

 Schemnitz district, further than is necessary to show the connexion 

 between the causes by which they are formed and the volcanic 

 activity of the area in which they occur. 



The fissures occupied by the Schemnitz lodes have a general 

 direction from N.N.E. to S.S.W. They traverse the so-called granite 

 and syenite, the " greenstone-trachyte," and the metamorphosed 

 Triassic strata, and also extend into the midst of the trachytes and 

 other rocks which surround the centre of the old volcanic mountain, 

 the most productive portions of the lodes appearing to be always 

 situated in the central masses of " greenstone-trachytes." According 

 to Yon Pettko there is evidence that in some cases the infilling of 

 the mineral veins is even of later date than the eruption of the basalts. 



The phenomena which we have described as being exhibited by 



the old volcano of Schemnitz find a very close parallel in the other 



great centres of igneous activity in Hungary and Transylvania. In 



each case we find a great circular or, sometimes, linear group of 



* Jahrbuoh der k.-k. Eeiehsanstalt, Bd. iv. (1855) p. 183. 



