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



The trachyte tuff is well seen in the Haisterbacher-thal, the other great valley which runs up 

 into the Siebcngebirge from Ober-Dollendorf, to the Stenzelberg and to Haisterbacherroth. At 

 tlie Langenberg it is loosely coherent, white, and pale red, contains much pumice, fragments of 

 slate ; and portions of jvood, in a soft state and of a brown colour, are not uncommon. Small 

 crystals of glassy felspar may be picked out of the earthy base, and the ruts in the road are often 

 coated with magnetic iron-sand, left by the rain that has washed the tuff. It contains numerous 

 masses of trachyte, m.<iny of which are quite fresh, and resemble those of the Drachenfels, Rosenau 

 and Stenzelberg; others unlike any found in situ. The tuff in this place lies in beds which are 

 nearly horizontal, but it exhibits that partial and irregular deviation from horizontality in short 

 distances, so often seen in sections of beds of gravel where there are alternations of sand, indi- 

 cating the deposition of the tuff to have taken place under the surface of water. Ascending the 

 valley at the foot of the Wirlberg to the Stenzelberg Kreuz, another section of the tuff is seen 

 covered by loess ; and Professor Noggerath states* that the tuff here was traversed by a vein of 

 opal jasper from two to three inches thick, and nearly vertical, very like the wood opal of Hungary • 

 it has long since been carried all away. Immediately above Ober Cassel a tuff is found which 

 differs considerably from any other in the district I have met with, in as much as it contains frag- 

 ments of basalt and scoriae, together with balls of clay ironstone. From its situation it is not im- 

 probable that this may be an after deposit of trachyte tuff, along with clay of the brown-coal for- 

 mation. 



Behind the village of Honnef a small knoll, called the Schwarzenerdenkopf, is formed of a con- 

 glomerate or rather breccia, to which I have found nothing like in any other part of the district. 

 It contains fragments of trachyte of different kinds, scoriaceous basalt, compact basalt, quartz, 

 crystals of felspar and hornblende. It is surrounded on all sides by grauwacke, and is of very 

 limited extent. It is altogether a very singular occurrence, and not very intelligible as to its 

 origin. 



Between the villages of Dambruch and Rott, on the northern slope of the Siebengebirge, con- 

 siderably within the region of the brown-coal formation, and four miles from the nearest point 

 where trachyte occurs, there are distinct beds of trachyte tuff lying between beds of clay and clay 

 ironstone. The same association of tuff with the brown-coal formation is found at Scheurn, Ut- 

 weiler, and Oehlinghoven. It is generally composed of smaller fragments than the tuff nearer to 

 the trachyte, but it contains masses of fresh hard trachyte. At Scheurn it contains impressions of 

 the leaves of trees, similar to those found in the brown-coal formation. 



The only spot on the left bank of the Rhine where I have found trachyte tuff is in a lane behind 

 the village of Muffendorf, between that place and Godesberg ; it is very similar to that near the 

 Drachenfels, and contains large masses of trachyte ■[•. 



It has been remarked by some who have described the Siebengebirge, 

 that the trachyte tuff always corresponds in composition with the trachyte 

 nearest to it, as if it were derived from a disintegrated trachyte, and were 

 thus a recomposed rock. I have looked for this correspondence of structure, 

 but could never satisfy myself that such is the fact. In the first place the tuff 

 is, in almost every case, in too earthy a state to enable one to say of what 

 particular species of trachyte it is composed; and in the next place, it occurs 

 generally surrounded by trachytes of various kinds ; and lastly, the masses or 



* Rheinland- fVestphalen, i. 139. f See Appendix III. p. 472. 



