TRUE VOLCANOES. 455 



ehatscheff), of Afran Karahissar (famous for the culture of the 

 poppy) and Menammed-kyoe in Phrygia, and of Kayadschyk 

 and Donanlar in Mysia, in which glassy felspar, with a great 

 deal of oligoclase, some hornblende, and brown mica are 

 mingled." 



guisbed in the fundamental mass. " These portions have been found 

 by I)r. Bothe, on chemical analysis in Mitscherlich's Laboratory, to 

 be oligoclase, corresponding exactly with the oligoclase of Danvikszoll 

 (near Stockholm) noticed by Berzelius." (Dechen, s. 340 346). The 

 Wolkenburg and the Stenzelberg are destitute of glassy felspar (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 Siebengebirge which treats of the relative age of trachyte-conglo- 

 merate and basalt conglomerate contains many new views (p. 405 461). 

 " With the more rare dykes of trachyte in the trachyte-conglomerates, 

 which prove that the formation of trachyte has still continued after the 

 deposit of the conglomerate (s. 413), are associated a great number of 

 basalt courses (s. 416). The basalt-formation extends decidedly into a 

 later basalt than the trachyte-formation, and the principal mass of the 

 basalt is here more recent than the trachyte. On the other hand a portion 

 of this basalt only, and not of all basalts (s. 323) is more recent than the 

 great mass of the brown-coal rocks. Both formations, the basalt and 

 the brown-coal rocks, run into each other in the Siebengebirge as well 

 as in many other places, and must be considered in the aggregate as 

 contemporaneous." Where very small crystals of quartz occur by 

 way of rarity in the trachytes of the Siebengebirge, as (according 

 to Noggerath and Bischof) in the Drachenfels and in the valley of 

 Rhondorf, they fill up cavities and seem to be of later formation (p. 361 

 and 370) ; caused perhaps by efflorescence of the sanidiue. On Chim- 

 borazo I have on one solitary occasion seen similar deposits of quartz, 

 though very thin, on the internal surfaces of the cavities of some very 

 porous, brick-red masses of trachyte at an elevation of about 17,000 

 feet (Humboldt, Gisement des Roches, 1823, p. 336). These fragments, 

 which are frequently mentioned in my journal, are not deposited in the 

 Berlin collections. Efflorescence of oligoclase, or of the whole funda- 

 mental mass of the rock may also yield such traces of disengaged silicic 

 acid. Some points of the Siebengebirge still merit renewed and perse- 

 vering investigation. The highest summit, the Lowenburg, represented 

 as basalt, seems, from the analysis of Bischof and Kjerulf, to be a do- 

 leritic rock (H. v. Dechen, s. 383, 386, 393). The rock of the little 

 Rosenau, which has sometimes been called Sanido-phyre, belongs, ac- 

 cording to G. Rose, to the first division of his trachytes, and is very 

 closely allied to many of the trachytes of the Ponga Islands. The 

 trachyte of the Drachenfels with large crystals of glassy felspar seems, 

 according to Abich's yet unpublished investigations, most nearly to 

 resemble the Dsyndserly-dagh which rises to a height of 8526 feet, to 

 the north of the great Ararat, from a formation of iiummulites under- 

 dipped by Devonian strata. 



