No. 6.] SANDBERGER — METALLIC VEINS. 349 



show very considerable quantities of sodic and potassic sulphate. 

 Whence the sulphates are derived, whether they should be 

 sought for in the numerous fluid- filled cavities of the quartz or 

 where else, has not yet been determined. The southern part 

 of the Black Forest is also rich in veins of baryta ; but the feld- 

 spars of the local gneisses and granites have not yet been examined 

 for baryta. Therefore the discussion of them must be postponed. 



The extremely rare occurrence of heavyspar in the geodes of 

 the phonolite of Oberschaffhausen in the Kaiserstuhl is very 

 easily explained. The presence of baryta, which A. Mitscherlich 

 observed in many sanidines, occurs also in this rock and the 

 decomposed nosean furnishes sulphates in sufficient quantity. 

 Therefore it cannot be wondered at that already in 1829 0. 

 Eisenlohr observed on mesotype wine-yellow heavyspars an inch 

 in size, and Schill repeats this observation. The small quantity 

 of heavyspar that occurs in the geodes of the phonolite proves 

 that for its separation a very active decomposition of the rock is 

 necessary, which a volcanic rock so comparatively new as the 

 phonolite has usually not yet undergone. Where such has 

 occurred, as for instance in the transformation of trachytic 

 rocks into alum-stone in the Hungarian volcanic zone in conse- 

 quence of the exhalation of sulphurous acid, the small contents 

 of baryta in the original rock has separated out in the form of 

 heavyspar, which is found in several places in the cavities of the 

 alum-stone. 



The abundance or the lack of veins of heavyspar in stratified 

 formations will therefore depend on how far the feldspar of the 

 primordial rocks used up in their formation was already decom- 

 posed at the time of their deposition. In the Permian and many 

 Triassic sandstones it is often still very fresh, in which case the 

 presence of baryta in the rock can be easily detected, and the 

 occurrence of veins of baryta at a considerable distance from the 

 primordial rocks be easily understood. Where this is not the 

 case the only possible supposition is that it was deposited by 

 springs. The feldspars in the sandstones of the Rhenish slate 

 plateau are very greatly decomposed, therefore veins of heavy- 

 spar occur very rarely in them ; for instance near Michelbach 

 and Oberrossbach in Nassau, Mittellach in the Bergischen, and 

 in a few Siegener veins. When the rock is lixiviated under an 

 increased pressure and temperature, as by the mineral springs at 

 Ems, the traces of baryta in the water prove that some of this 



