320 COSMOS. 



a metallic lustre : other,? are basaltic and full of small gra- 

 nules of olivine. When we had thus ascended to the upper 

 surface of the lava-stream at a perpendicular elevation of 

 711 feet, we turned to the white ash cone, on which, from 

 its great steepness, we could not but fear that during fre- 

 quent and rapid slips we might be seriously wounded by the 

 rugged lava. The upper margin of the crater, on the south 

 western part of which we placed the instruments, forms 

 a ring of a few feet in width. We carried the barometer 

 from the margin into the oval crater of the truncated cone. 

 At an open fissure air streams forth of a temperature of 

 200'6. We now stood 149 feet in perpendicular height 

 below the margin of the crater ; and the deepest point of 

 the chasm, the attainment of which we were compelled to 

 give up on account of the dense sulphurous vapours, ap- 

 peared to be only about twice this depth. The geognostic 

 discovery which had the most interest for us, was the find- 

 ing of several white fragments, three or four inches in dia- 

 meter, of a rock rich in felspar baked into the black basaltic 

 lava. I regarded these at first" as syenite, but from the 



12 " M. Bonpland and myself were particularly astonished at finding, 

 encased in the basaltic, lithoid and scorified lavas of the volcano of 

 Jorullo, white or greenish white angular fragments of Syenite, com- 

 posed of a little amphibole and a great quantity of lamellar felspar. 

 Where these masses have been split by heat, the felspar has become 

 filamentous, so that the margins of the crack are united in some places 

 by fibres elongated from the mass. In the Cordilleras of South 

 America, between Popayan and Almaguer, at the foot of the Cerro 

 Broncoso, I have found actual fragments of gneiss encased in a trachyte 

 abounding in pyroxene. These phenomena prove that the trachytic 

 formations have issued from beneath the granitic crust of the globe. 

 Analogous phenomena are presented by the trachytes of the Siebenge- 

 Mrge on the banks of the Khine, and by the inferior strata of Phono- 

 lite (Porphyrschiefer) of the Biliner Stein in Bohemia." (Humboldt, 

 Essai Geognostique sur le Gisement des Roches, 1823, pp. 133 and 339. 

 Burkart also (Aufenthalt und Reisen in Mexico, Bd. i, s. 230) detected 

 enclosed in the black lava, abounding in olivine, of Jorullo : " Blocks 

 of a metamorphosed syenite. Hornblende is rarely to be recognized 

 distinctly. The blocks of syenite may certainly furnish an incontro- 

 vertible proof, that the seat of the focus of the volcano of Jorullo is 

 either in or below the syenite, which shows itself in considerable 

 extent, a few miles (leguas) further south, on the left bank of the Rio 

 de las Balsas, flowing into the Pacific Ocean." Dolomieu, and, in 1832, 

 the excellent geognosist, Friedrich Hoffmann, found in Lipari, neai 



