No. 6.] SANDBERGER — METALLIC VEINS. 351 



nomena occur in the large pegmatite vein of the Gleisinger Rock 

 in the Fichtelgebirg, in whose fissures violet-blue fluorspar and 

 iron-mica occur together in exact proportion as the dark mica 

 (rich in iron)* and the oligoclase are decomposed. Here, as I 

 already advanced in 1865, there remains no doubt that the 

 origin of the fluorspar depends upon the reaction of the potassic 

 fluoride, that is derived from the mica, with the calcic carbonate 

 from the oligoclase. The spathic iron, which occurs in scales in 

 the fine crevices, is ferric oxyd that separated out of the mica 

 at the same time ; and upon the more complete decomposition 

 of the rock it has even concentrated itself into a deposit of schis- 

 tose very pure spathic iron, which has been worked for some 

 time past. Baryta does not occur here ; for the feldspar (com- 

 pletely changed into pinitoid) that furnished the lime is in its 

 fresh condition an oligoclase free from baryta, which contains, 

 according to Gerichten : 



Si0 2 61.36 



A1 2 3 22.25 



Fe 2 O s 1.60 



CaO 1.10 



Mg Trace 



Na 2 11.06 



K 2 2.07 



The orthoclase of the rock is very much reddened, but not yet 

 changed into pinitoid. 



In the Black Forest the veins richest in fluorspar are those in 

 decomposed gneiss, e. g., Friedrich Christian in Schapbach, 

 Teufelsgrund in the Muensterthal, Stephanie near Schoenau, 

 Maus near Todtnau and Neuglueck near St. Blasien ; numerous 

 other mines also contain the mineral, although not in such quan- 

 tity as in the Friedrich Christian, where 714 cwt. were mined 

 in the years 1853-1857 alone. The production continued until 

 1876 in the so-called Fluorspar Shaft, where it was six feet wide. 

 Fluorspar is one of the minerals that with especial clearness 

 prove the co-operation of organic subtances in the formation of 

 ore-deposits. The crystals are almost always coloured by organic 

 dye-stuffs, sometimes dark violet (Schoenau), pale violet (Muen- 

 sterthal, Schapbach), light sea-green (Schapbach, Todtnau), and 



* The clear potash-mica that is also present undergoes decomposi- 

 tiononly much later. 



