440 Mr. Horner on the Geology of the Environs of Bonn. 



The substances which in Hungary and Central Prance accompany trachyte, 

 viz. pitchstone, obsidian, pearlstone, and pumice, do not occur in the Siebenge- 

 birge, if we except some portions of the latter that are met with occasionally 

 in the trachyte tuff. It has in no instance flowed as a stream, nor do any of 

 the hills in the Siebengebirge show any signs of a crater. 



The ground is so covered with vegetation that it is very rarely that the different rocks can be 

 seen in contact. In one place only have I been able to discover the trachyte and grauvvacke to- 

 gether : this is behind the village of Rhondorf, at the Bruckersberg, where trachyte rises under grau- 

 wacke strata, inclined at a high angle, bent and broken, and both are covered by a thick unstratified 

 mass of loess. The trachyte is much decomposed, but at a short distance it is tolerably fresh. 



TTacht/te, 



Gramvackc. 



Loess. 



Alluvial Soil. 



When the water in the Rhine is low, trachyte rocks may be seen stretching from the foot of the 

 Drachent'els a considerable way across the river ; but it has not been found in any part of this 

 district on the left bank of the Rhine, except at Borkum, about tliree miles west by south of Meh- 

 lem : there it is of a grayish white colour, is composed of white felspar and minute portions of green 

 hornblende, with imbedded crystals of glassy felspar. It forms a small hill, surrounded on all 

 sides by grauwacke slate, but the junction cannot be seen*. 



Trachyte Tuff. 



A considerable part of the Siebengebirge is covered with a tufff consisting 

 of angular fragments of trachyte, generally in an earthy state, especially as 



* Von Oeynhausen and Von Dechen, in Hertha, xii. 432. 



•\ I adopt the term trachyte tuff in preference to that of conglomerate, by which it is usually 

 called by the German geologists, because, in England, conglomerate would generally be under- 

 stood to mean a congeries of water-worn fragments of various substances. 



