424 COSMOS. 



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

 crystals in trachytes of Mont Dore* never in the Phlegrsean 

 Fields together with hornblende ; nor is leucite, of which 

 last, however, Hoffmann collected some pieces on the Lago 

 Averno (on the road to Cuma3), while I found some "on the 

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

 cite ophyr in loose 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 cry&tals of glassy feldspar, and a profusion of small 

 snow-white crystals of oligoclase. The latter arc frequently 

 overspread with the glassy feldspar in regular order, and 

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

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

 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 dc PAuvergne, in the Mem. de la Soc. 

 GcoL de France, 2me Serie, t. i., partie i., 1844, p. G9. 



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

 are described by Gustav Rose in Fried. Hoffmann's Geognostischen Bco- 

 bachtungcn, 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, 

 Monographic des Vestivs, 1857, s. 519-522, tab. viii. The trachyte of 

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

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

 (s. 528), but no leuche. 



J 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 Natwhistorischen Vereines der JPreuss, 

 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 cry stalline particles may be distinguished in 

 the fundamental mass. " These portions have been found by I)r. 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 n 

 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). " \Vith 

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



