true volcanoes. 443 



Hornblende and Attgite. 



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 the 

 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 -ZEtna). I have found hornblende, 

 though not in large quantities, in the trachytes of the volca- 

 noes of Cotopaxi, Rucu-Pichincha, Tungurahua, and Anti- 

 sana, along with augite and oligoclase, but scarcely ever 

 along with these two minerals on the slope of the Chimbo- 

 razo up to a height of more than 19,000 feet. Among the 

 many specimens which I brought from Chimborazo, horn- 

 blende is recognized only in two, and even then in small 

 quantity. In the eruptions of Vesuvius in the years 1822 



which are penetrated by auriferous veins, along with the sanidine con- 

 tain also grains of brownish quartz. The small inclosures of grains of 

 obsidian and glassy feldspar being, on the whole, rare in the volcanic 

 rocks at the Cerro de las Navajas, and in the Valle de Santiago, so 

 rich in basalt and pearl-stone, which is traversed in going from Valla- 

 dolid 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 Baromhtr., 

 p. 327, No. 366, and Essai Gcognostique 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 particularly in the neighborhood 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 explicatifd'une Carte Geogn. des Etats Unis, 1855, p. 3.) 



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

 is not 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 toward 

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

 confounded with the older stream, described by Strabo, near Montag- 

 none and Rotaro ( Cosmos, see above, p. 252, 399). Glassy feldspar is 

 not only rare in the trachytes of Cotopaxi and other volcanoes of the 

 Cordilleras generally, but it is equally so in the subten-anean pumice 

 quarries at the foot of the Cotopaxi. What was formerly described as 

 sanidine are crystals of oligoclase. 



