434 COSMOS. 



gite,* a doleritic trachyte : .^iEtna, Stromboli ; and, according 

 to the admirable works on the trachytes of the Antilles by 

 Charles Sainte-Claire Deville, the Soufriere de la Guade- 

 loupe, as well as the three great cirques which surround the 

 Pic de Salazu, on Bourbon." 



Sixth Division. — " The ground mass, often of a gray color, 

 in which crystals of leucite and augite lie imbedded, with 

 very little olivin: Vesuvius and Somma; also the extinct 

 ^volcanoes of Vultur, Rocca Monfina, the Albanian Hills, and 

 Borghetto. In the older mass (for example, in the wall and 

 paving stones of Pompeii) the crystals of leucite are more 

 considerable in size and more numerous than the augite. In 

 the present lavas, on the contrary, the augites predominate, 

 and the leucites are, on the whole, very scarce, although the 

 lava stream of the 22d of April, 1845, has furnished them 

 in abundance. t Fragments of trachytes of the first division, 



* Sartorius von Waltershausen, who has for many years carefully 

 investigated the trachytes of -^tna, makes the following important 

 observations: "The hornblende there belongs especially to the older 

 masses — the green-stone veins in the Val del Bove, as well as the 

 white and red trachytes, which form the ground mass of vEtna in 

 the Serra Giannicola. Black hornblende and bright yellowish-green 

 augite are there found side by side. The more recent lava streams, 

 from 166i) (especially those of 1787, 1809, 1811, 1819, 1832, 1838, and 

 1842), show augite, but no hornblende. The latter seems to be gen- 

 erated only after a longer period of cooling" (Waltershausen, Ueber 

 die vulkanischen Gesteine von Sicilien und Island, 1853, s. 111-114). In 

 the augitiferous trachytes of the fourth division, in the chain of the 

 Andes, along with the abundant augites, I have indeed sometimes 

 found none, but sometimes, as at Cotopaxi (at an elevation of 14,068 

 feet) and- at Rucu-Pichincha, at a height of 15,304 feet, distinct black 

 hornblende crystals in small quantities. 



t See Pilla, in the Comptes rendus de VAcad. des Sc, t. xx., 1845, 

 p. 324. In the leucite crystals of the Rocca Monfina, Pilla has found 

 the surface covered with worm tubes {serpulm), indicating a submarine 

 volcanic formation. On the leucite of the Eifel, in the trachyte of 

 the Burgberg, near Rieden, and that of Albano, Lago Bracciano, and 

 Borghetto, to the north of Rome, see above, page 224, note *. In the 

 centre of large crystals of leucite, Leopold von Buch has generally 

 found the fragment of a crystal of augite, round which the leucite 

 crystallization has formed, "a circumstance which, considering the 

 ready fusibility of the augite, and the infusibility of the leucite, is 

 somewhat singular. More frequently still are fragments of the funda- 

 mental mass itself inclosed like a nucleus in leucite porphyry." Oli- 

 vin is likewise found in lavas, as in the cavities of the obsidian which 

 I brought from the Cerro del Jacal, in Mexico {Cosmos, vol. i., p. 266, 

 note %), and yet, strange to say, also in the hypersthene rock of Elf- 

 dal (Berzelius, Sechster Jahresbericht, 1827, s. 302), which was long 

 considered to be syenite. A similar contrast in the nature of the 

 places where it is found is exhibited by oligoclase, which occurs in the 



