ON ITS EXTERIOR. VOLCANOES. 439 



no white (mostly biaxial) potassa-mica, but only dark- 

 coloured (mostly uniaxial) magnesia-mica, appears to 

 have been produced ; and that this exclusive occurrence 

 of magnesia-mica extends to many other eruptive and 

 plutonic rocks, basalt, phonolite, syenite, syenite-slate, 

 and even to granitite ; while granite proper contains 

 simultaneously white potassa-mica and black or brown 

 magnesia-mica. ( 617 ) 



Glassy Felspar. 



This kind of felspar, which plays so important a part 

 in the activity of European volcanoes, in the trachytes 

 of the first and second divisions (for example, at Ischia, 

 in the Phlegrsean Fields, and in the Siebengebirge near 

 Bonn), is probably wholly wanting in the New Con- 

 tinent in the trachytes of active volcanoes ; which is the 

 more striking, inasmuch as sanidine (glassy felspar) be- 

 longs essentially to the argentiferous non-quartzose 

 Mexican porphyries of Moran, Pachuca, Villalpando, 

 and Acaguisotla, of which the first are connected with 

 the obsidians of Jacal. ( 618 ) 



Hoi^nblende and Augite. 



In the preceding account of the characteristics of the 

 six different divisions of trachytes, it was remarked that 

 the same mineral species which appear as essential ingre- 

 dients or constituents in some divisions (as, for example, 

 hornblende, in the third division, or Toluca rock), ap- 

 pear in other divisions (e.g. in the fourth and fifth, 

 the Pichincha and Etna rocks) only sporadically. I 

 have found hornblende, though not abundantly, in the 

 trachytes of the volcanoes of Cotopaxi, Euca-Pichincha, 



