474 COSMOS. 



of the Meronitz, of the marly Kausawer Mountain and espe 

 cially of the Camay er summit 91 of the central Bohemian 

 chain j more rarely in the phonolite, 93 as well as in the dole- 

 rite of the Kaiserstuhl near Freiburg. It is remarkable 

 that in the trachytes and lavas of both continents not only 

 no white (chiefly bi-axal) potash-mica is observable, but 

 that it is entirely dark-coloured (chiefly uni-axal) magnesian- 

 mica, and that this exceptional occurrence of the magnesia- 

 mica is extended to many other rocks of eruption and plu- 

 tonic rocks, such as basalt, phonolite, syenite, syenitic- 

 slate, and even granitite, while the granite proper contains 

 at one and the same time, white alkaline-mica and black or 

 brown magnesia-mica. 93 



GLASSY FELSPAR. 



This kind of felspar, which plays so important a part 

 in the action of European volcanoes ; in the trachytes of 

 the first and second division (for example, on Ischia, in the 

 Phlegrsean Fields, or the Siebengebirge near Bonn), is proba- 

 bly entirely wanting in the New Continent, in the trachytes 

 of active volcanoes. This circumstance is the more striking 

 as sanidine (glassy felspar) belongs essentially to the argen- 

 tiferous, non-quartzose Mexican porphyries of Moran, Pa- 

 chuca, Villalpando and Acaquisotla, the first of which are 

 connected with the obsidians of Jacal. 94 



91 See the Berymannisches Journal, von Kohler und Hofmann, 5ter 

 Jahrgang, Bd. i, 1792, s. 244, 251, 265. Basalt rich in mica, as on 

 the Gamayer summit in the Bohemian centre mountains, is a rarity. I 

 visited this part of the Bohemian central range in the summer of 1792, 

 in company with Carl Freiesleben, afterwards my companion in my 

 Swiss tour, who has exercised so great an influence over rny geological 

 and mining education. Bischof doubts all production of mica by the 

 igneous method, and considers it a metamorphic product by the moist 

 method. See his Lehrbuch der ckem. und physikal. Geologic, Bd. ii, 

 s. 1426, 1439. 



92 Jenzsch, JBeitraye zurKenntn/ss der Pkonolithe, in der Zeitschrift der 

 Dcutschen Geoloyischen Gesellsckaft, Bd. viii, 1856, s. 36. 



93 Gustav Rose, Ueber die zur Granitgruppe yehoriyen Gzbirysarten, 

 ibi'.l,, Bd. i, 1849, s. 359. 



!4 The porphyries of Moran, Real del Monte and Regla (the latter 

 celebrated for the rich silver mines of the Veta Biscayna, and 

 the vicinity of the obsidians and pearlstones of the Cerro del Jacal 

 and the Messerberg, Cerro de las Navajas), like almost all the metal- 

 liferous porphyries of America, are quite destitute of quartz (ou 

 and other analogous phenomena in Hungary, see Humboldt, 



