Mr. Horner on the Geology of the Environs of Bonn. 441 



regards the felspar, for the most part loosely aggregated, but sometimes so 

 compact as to be quarried for various economical purposes. The fra^-ments 

 are usually small, seldom larger than hazel-nuts or small beans, and sometimes 

 so fine, that when the stone is white and compact, it is very like a coarse chalk. 

 It occurs grayish white, yellow, and brown of various tints, and sometimes of a 

 reddish colour. It frequently contains masses or balls of hard trachyte, vary- 

 ing in diameter from a few inches to two feet and upwards, and fragments of 

 slate, usually angular, but sometimes rounded. Except in one situation, which 

 I shall afterwards mention, I never found any basalt fragments in the tuff", 

 an important fact as regards the history of these volcanic regions. They 

 were formerly stated to have occurred, but later observations have shown 

 that what was taken for basalt is a very compact black slate. I searched for 

 basalt in those places where it was said to occur, but never found any. 



The trachyte tuff generally occupies tlie intervening valleys, and is found at considerable ele- 

 vations, but none of the higher summits are composed of it : they are either of trachyte or basalt. 

 Its extent and mode of surrounding the trachytic and basaltic hills will be best understood by an 

 inspection of the Map, but I shall mention some of the spots where it may be most distinctly seen. 



The road from Konigswinter into the Wintermiihlen-thal goes across the extremity of a ridge, 

 which descends from the Drachenfels, apparently wholly composed of trachyte tuff, and at a place 

 called the Holle it is cut through to the depth of forty feet. Here it is of a brown colour, and 

 seems to contain much iron. It is traversed in all directions by lines like the decomposing crusts 

 of two surfaces united, which sometimes give a false notion of stratification : in many places it is 

 decomposed in concentric layers, like basalt, and it contains numerous masses of trachyte and 

 slate. It is traversed by basalt dikes, which will be afterwards described. A tuff similar to this 

 occurs between the Lohrberg and Scheerkopf, where it is traversed by the trachyte dike, men- 

 tioned p. 439, and it is here at an elevation of about 900 feet. The tuff in these two places is 

 different from any that occurs elsewhere in the district. After passing the Holle, and arriving at 

 Quegstein, at the entrance of the valley, a white, very loosely coherent tuff, including fragments of 

 slate, rests on a compact sandstone of the brown-coal formation, and is covered by loess : specimens 

 may be obtained of the junction of the tuff' and sandstone. In the museum of the University of 

 Bonn there are specimens of tuff" from this place with impressions of the leaves of trees similar to 

 those which are found in the brown-coal formation. In the immediate vicinity of this spot there 

 was a knoll of basalt, which contained fragments of slate converted into jasper, of which several 

 specimens may be seen in the museum ; but the hard rock has been entirely removed for mending 

 the roads. Higher up the valley, the tuff", preserving its white colour, becomes more compact, and 

 at the Ofenkulenberg, a ridge descending from the Wolkenburg, it is quarried. It is much used 

 for building ovens, from its property of resisting heat, and is thence called Backofenstein ; and from 

 being easily worked, masses are hollowed out to form troughs. In the road leading to the quarry 

 jt is traversed by a dike of basalt; and Mr. Noggerath* says, that at this place impressions of 

 leaves have been found in the tuff". In the Keilsbrunnen Steig, near the Ofenkulenberg, a similar 

 tuff" is traversed by dikes of basalt. 



Rheinland-Westphalen, i. 135. 



