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



One of the most remarkable volcanic eruptions of this neighbourhood 

 occurs at Siegburg, a small town in the northern part of the district. The 

 small river S'leg winds through an extensive alluvial plain, and in the midst of 

 this, entirely detached from the adjoining hills, three conical eminences rise up, 

 one of them quite insulated, the other two connected together by a low ridge. 

 At the foot of the insulated cone, which is 211 feet high, the town of Sieg- 

 burg is situated, and on its summit there is a vast building, formerly a mo- 

 nastery, now a public Lunatic Asylum, which forms a very conspicuous object 

 for many miles round. The principal mass of these three hills is a tuff of a 

 yellowish brown colour, in some places compact, but more usually of a loose 

 texture ; in point of structure very like trachyte tuff, but substituting the con- 

 stituent parts of basalt for trachyte as the material ; and it is used for similar 

 purposes, viz. the construction of ovens and fireplaces. Its component parts are 

 for the most part decomposed basalt, but sometimes the fragments are fresh ; 

 many are in the state of scoriae, and there are numerous fragments of grau- 

 wacke slate. It is often penetrated by calcareous spar, seemingly introduced by 

 subsequent infiltration ; and it contains numerous fragments of dicotyledonous 

 wood, generally brown like lignite, sometimes fossilized by calcareous spar, at 

 others highly charged with iron ; balls of clay ironstone are not unfrequent. 

 Mr. Lyell, who visited this spot with me, remarked the great resemblance of this 

 tuff to the volcanic tuff of the PhlegraBan Fields and of Ischia. The two con- 

 nected hills are called the Grimprich and Wolsberg, and are 189 and 230 feet 

 high. From the former stands out a mass of basalt at the summit, a part of a 

 dike from the sides of which the tuff" has been washed away, portions of it 

 being still left adhering; the main part of the dike is a compact black basalt, 

 but towards the sides it becomes scoriform ; it may be traced to the bottom of 

 the hill. On the south-east side the tuff lies in beds dipping from the hill, 

 that is, with a slope nearly parallel to its present surface. The basalt contains 

 nests of arragonite in well-defined crystals. In the Wolsberg there are large 

 quarries where the tuff may be seen in all its varieties, being sometimes so 

 hard as to require to be blasted. On the south side, and just above the Sieg, 

 it is traversed by a double dike of basalt, each branch about a foot wide, the 

 tuff between being very different from that on the outside, appearing like 

 cinders that had run together, part being in a twisted form, as is often seen 

 when a viscid ropy stream of lava has cooled ; the basalt of the dikes is also 

 scoriform. No place can show in a more distinct manner the similarity of an 

 undoubted lava to basalt occurring in situations where none of the usual ap- 

 pearances of active volcanos exist; for the basalt of these two cones could not 



