TBUE VOLCANOES. 475 



HOR^BLEN'DE AND AUGITE. 



In this account of the characteristics of six different divi- 

 sions of the trachytes, it has been already observed how the 

 same minerals which occur as essential elements of commix- 

 ture (for example, hornblende in the third division, or tha 

 Toluca rock), appear in other divisions in a separate or spo- 

 radic condition (as in the fourth and fifth divisions, in the 

 rock of Pichincha and of Etna). I have found hornblende,, 



(jeognostique sur le Gisement des Roches, pp. 179 188 and 

 190 193). The porphyries of Acaquisotla, however, on the road 

 from Acapulco to Chilpanzingo, as well as those of Villalpando to 

 the North of Guauaxuato, which are penetrated by auriferous veins, 

 along with the sauidine contain also grains of brownish quartz. 

 The small inclosures of grains of obsidian and glassy felspar being 

 on the whole rare in the volcanic rocks at the Cerro de las Navajas, 

 arid in the Valie de Santiago, so rich in basalt and pearl -stone, 

 which is traversed in going from Valladolid to the volcano of Jorullo, 

 I was the more astonished at finding at Capula and Pazcuaro, and 

 especially near Yurisapundaro, all the ant-hills filled with beautifully 

 shining grains of obsidian and sanidine. This was in the month of 

 September, 1803 (Nivellement barometr. p. 327, No. 366, and Essai 

 geoynost. sur le Gisement des Roches, p. 356). I was amazed that Such 

 small insects should be able to drag the minerals to such a distance. 

 It has given me great pleasure to find that an active investigator, M. 

 Jules Marcou, has observed something exactly similar. " There exists," 

 he says, " on the high plateaux of the Rocky Mountains, and particu- 

 larly in the neighbourhood of Fort Defiance (to the west of Mount 

 Taylor), a species of ant which, instead of using fragments of wood 

 and vegetable remains for the purpose of building its dwelling, employs 

 only small stones of the size of a grain of maize. Its instinct leads it 

 to select the most brilliant fragments of stones, and thus the ant-hill 

 is frequently filled with magnificent transparent garnets and very pure 

 grains of quartz." (Jules Marcou, Resume explicatif d'une Carte geogn. 

 des Etats-unis, 1855, p. 3.) 



Glassy felspar is very rare in the present lavas of Vesuvius, but 

 this is uot the case in the old lavas, for instance in those of the eruption 

 of 1631, where it occurs along with crystals of leucite. Sanidine is 

 also found in abundance in the Arso lava-stream, from Cremate towards 

 Ischia, of the year 1301, without any leucite ; but this must not be con- 

 founded with the older stream, described by Strabo, near Montag* 

 none and Rotaro (Cosmos, see above, pp. 265, 427). Glassy fel- 

 spar is not only rare in the trachytes of Cotopaxi and other vol- 

 canoes of the Cordilleras generally, but is equally so in the subterranean 

 pumice-quarries at the foot of the Cotopaxi. What was formerly de- 

 scribed aa sanidine are crystals of oligoclaae. 



