360 THE CANADIAN NATURALIST. [Vol. viii. 



Cobalt, the most easily detected metal, is absent from none of 

 the micas examined by me ; most of them contain copper also, 

 often indeed in comparatively large quantity, e. g., that from 

 Petersthal, Zindelstein near Hammereiseobrech and Schapbach 

 in the Black Forest, Damm and Hoerstein in the Spessart, and 

 it is then usually accompanied by bismuth. Nickel is rarer, but 

 it also has been found, for instance, in the mica of the garnet- 

 gneiss from Wittichen and that of the kersantite of Nassau. 

 Arsenic was detected only in that from Schapbach, Schiltach and 

 Hoerstein. The micas from the granitic region of Wittichen 

 and Schapbach contain traces of silver, but no lead. This, how- 

 ever, has been very clearly detected in the mica of the gneiss of 

 Schapbach. 



There are in the gneiss, as well as in the region of hornblende- 

 slates, ore-deposits whose sulphurets are so intermingled with 

 silicates, micas, cordierite and feldspars that they can hardly be 

 considered otherwise than as having been separated out at the 

 time of formation of the gneiss. To this class, in my opinion, 

 belong, for instance, those of Bodenmais in Bavaria, Todtmoos 

 in Baden and Orijaerfi in Finnland, which principally contain 

 magnetic pyrites, blende, copper pyrites and galena, and those of 

 Modum in Norway and Tunaberg in Sweden, containing cobal- 

 tine and copper pyrites. In these ore-deposits occur again all 

 the heavy metals that were found in the micas of the gneisses. 

 In the case of a great excess of iron and lack of arsenic, only 

 magnetic pyrites occurs, which, however, usually carries with it 

 the remaining metals of the iron group. When, at the same 

 time, as at Bodenmais, it contains but little nickel and cobalt, 

 they can still be detected, and can be quantitatively determined 

 in the pyrites of Kleofa (3.044 Ni, 0.094 Co, Berzelius), Todt- 

 moos (Ni 1.82, Co 0.48, Hilger) and Horbach near St. Blasien 

 (Ni 3.86, Rammelsberg, 11.2, Knop) ; and in this latter case 

 are technically useful. Whenever arsenic occurs in these ore- 

 combinations, the cobalt concentrates itself in the form of 

 cobaltine, while the iron and copper form sulphurets free from 

 arsenic. 



The metallic veins in the gneiss region vary greatly in regard 

 to their filling, which I seek to explain only by the chemical 

 composition of the dark mica contained in the gneiss in question. 

 In the gneiss of the Spessart, whose dark mica contains cobalt 

 and arsenic along with copper and bismuth, occur the well-known 



