424 COSMOS. 



Grande Cascade). Augite is but very rarely found in small 

 crystals in trachytes of Mont Dore* — never in the Phlegraean 

 Fields together with hornblende ; nor is leucite, of which 

 last, howev.er, Hoffmann collected some pieces on the Lago 

 Averno (on the road to Cumae), while I found some on the 

 slope of the Monte Nuovoj (in the autumn of 1822). Leu- 

 cite ophyr in lo.ose fragments is more frequent in the island 

 of Procida and the adjoining Scoglio di S. Martino." 



Second Division. — "The ground mass contains some de- 

 tached crystals of glassy feldspar, and a profusion of small 

 snow-white crystals of oligoclase. The latter are frequently 

 overspread with the glassy feldspar in regular order, and 

 form a covering about the feldspar, as is so frequently seen 

 in G. Rose's granitite (the principal mass of the Eiesenge- 

 birge and Isergebirge, consisting of granite with red feldspar, 

 particularly rich in oligoclase and magnesian mica, but with- 

 out any white potash mica). Hornblende and mica, and in 

 some modifications augite, occasionally appear in small quan- 

 tity. To this division belong the trachytes of the Drachen- 

 fels and of the Perlenhardt, in the Siebengebirge,j: near Bonn, 



* Rozet, Sur les Montagues de I'Auvergne, in the Man. de la Soc. 

 Gcol. de France, 2me Serie, t. i., partie i., 1844, p. 69. 



t Fragments of leucite ophyr, collected by me at the Monte Nuovo, 

 are described by Gustav Rose in Fried. Hoffmann's Geognostischen Beo~ 

 hachtungen, 1839, s. 219.. On the trachyte of the Monte di Procida of 

 the island of the same name, and the rock of San Martino, see Roth, 

 Monographie des Vesuvs, 1857, s. 519-522, tab. viii. The trachyte of 

 the island of Ischia contains in the Arso, or stream of Cremate (1301), 

 vitreous feldspar, brown mica, green augite, magnetic iron, and olivin 

 (s. 528), but no leucite. 



X The geologico-topographical conditions of the Siebengebirge near 

 Bonn have been developed with comprehensive talent and great exact- 

 ness by my friend H. von Dechen, director of mines, in the 9th annual 

 volume of the Verhandlungen des Naturhistorischen Vereines der Preuss, 

 Rheinlande, und Westphalens, 1852, s. 289-567. All the chemical analy- 

 ses of the trachytes of the Siebengebirge which have hitherto appeared 

 are there collected (p. 323-356) ; mention is also made of the trachytes 

 of the Drachenfels and Rottchen, in which, besides the large crystals 

 of sanidine, several small crystalline particles may be distinguished in 

 the fundamental mass, " These portions have been found by Dr. Bothe, 

 on chemica lanalysis in Mitscherlich's laboratory, to be oligoclase, 

 corresponding exactly with the oligoclase of Danvikszoll (near Stock- 

 holm) noticed by Berzelius." (Dechen, s. 340-346.) The Wolken- 

 burg and the Stenzelberg are destitute of glassy feldspar (s. 357 and 363), 

 and belong, not to the second division, but to the third ; they contain a 

 Toluca rock. That section of the geological description of the Sie- 

 bengebirge which treats of the relative age of trachyte conglomerate 

 and basalt conglomerate contains many new views (p. 405-461). " With 

 the more rare dikes of trachyte in the trachyte conglomerates, which 



