442 cosmos. 



of the Meronitz, of the marly Kausawer Mountain, and espe- 

 cially of the Gamayer summit* of the central Bohemian 

 chain ; more rarely in the phonolite, "j" 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-colored (chiefly uni-axal) magnesian mica, 

 and that this exceptional occurrence of the magnesia mica is 

 extended to many other rocks of eruption and Plutonic 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.f 



Glassy Feldspar. 



This kind of feldspar, 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 Phlegraean 

 Fields, or the Siebengebirge near Bonn), is probably entirely 

 wanting in the New Continent, in the trachytes of active vol- 

 canoes. This circumstance is the more striking, as sanidine 

 (glassy feldspar) belongs essentially to the argentiferous, non- 

 quartzose Mexican porphyries of Moran, Pachuca, Villalpan- 

 do, and Acaquisotla, the first of which are connected with 

 the obsidians of Jacal.§ 



* See the Bergmdnnisches 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, afterward my companion in 

 my Swiss tour, who has exercised so great an influence over my geo- 

 logical 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 Chem. und Physikal. Geologie, 

 bd. ii., s. 1426, 1439. 



t Jenzsch, Beitrage zur Kenntniss der Phonolithe, in der Zeitschrift 

 der Deutschen Geologischen Gesettschqft, bd. viii., 1856, s. 36. 



X Gustav Rose, Ueber die zur Granitgruppe gehbrigen Gebirgsarten, in 

 derselben Zeitschrift, bd. i., 1849, s. 359. 



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

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

 cinity of the obsidians and pearl-stones of the Cerro del Jacal and the 

 Messerberg, Cerro de las Navajas), like almost all the metalliferous 

 porphyries of America, are quite destitute of quartz (on these and oth- 

 er analogous phenomena in Hungary, see Humboldt, Essai Geognos- 

 tique sur le Gisement des Roches, p. 179-188, and 190-193). The por- 

 phyries of Acaquisotla, however, on the road from Acapulco to Chil- 

 panzingo, as well as those of Villalpando to the north of Guanaxuato, 



