Mr. Horner on the Geology of the Environs of Bonn. 445 



columns lying in an inclined position, and separated from each other as if there had been an in- 

 termediate part reduced to an earthy state by decomposition. The basalt itself is very fresh, and 

 contains many simple minerals, especially olivine in rounded granular masses, sometimes as large 

 as a turkey's egg, calcareous spar, hornblende, mesotype, hyacinth, and magnetic iron. In the 

 upper part of the quarry there is a basalt conglomerate which includes fragments of wood, and 

 above that a pretty thick bed of loess. When the water in the Rhine is low the columnar basalt 

 is seen extending a considerable way into the stream, and before a great part of it was removed 

 it rendered the navigation here difficult and dangerous. Eruptions of columnar basalt may also 

 be seen at Rolandseck, at Dechelsberg near Nieder Bachem, at Liihnsberg near Muffendorf, and 

 near Kessenich ; and the conical hill of Godesberg, which, crowned with its ruined castle, forms 

 so conspicuous a feature in the landscapes of this neighbourhood, is composed of the same rock. 



Basalt Dikes. 

 These are of frequent occurrence in the Siebengebirge. 



In the Keilsbrunnen quarry, not far from the Lowenburg, a dike of amygdaloidal trap, about 

 twelve feet thick, traverses trachyte ; and one traversing the trachyte tufFin the immediate neigh- 

 bourhood, from its composition and direction, seems to be a prolongation of the same. Not far 

 from hence, at the Lowenburg Tranke, the trachyte is traversed by a dike of amygdaloidal trap 

 about two feet wide, the trachyte being much decomposed. In the lane leading from Konigswinter 

 to the Keilsbrunnen quarry, between the Hirschberg and Wolkenburg, the trachyte tuff is traversed 

 by four dikes in a space of about seventy- five feet. They are narrow, but their dimensions and 

 their relations to the substance they cut through cannot be well seen from the covering of soil. 

 Near the Stenzelberg Kreuz the trachyte tuff is cut by a dike which probably is connected with the 

 basaltic hill, the Wirlberg, close by, and which probably has also sent out the dike in the opposite 

 direction, which is seen near the Thier Garten. In a road leading over the hills due east of Hon- 

 nef, near Steinsbusch, a dike of basalt cuts the grauwacke strata ; there is no change produced in 

 the latter either in regard to disturbance of the regularity of the stratification or the texture of the 

 stone ; but at no great distance, at the Goldnen Kistchen, a mass of basalt in contact with the 

 grauwacke changes it into a hard substance like jasper, which splits into prismatic fragments. 



In the Konigswinter valley, and in the road to the Ofenkulenberg quarry, and not far from it, a 

 white trachyte tuff is traversed by a trap dike about six feet wide ; the tuff is considerably har- 

 dened at and near the junction, and splits into prismatic fragments. The dike is a dark amygda- 

 loidal basalt with olivine ; but there seems to have been an intermixture with the tuff towards the 

 sides, where it is so changed as scarcely to be recognisable from basalt in detached specimens. 

 Where it is of a dark colour it separates into thin plates like a coarse slate, parallel to the direction 

 of the dike. The tuff in the Konigswinter Holle, described p. 441. is traversed by two dikes of 

 about two feet in width, which intersect each other ; the one being thrown considerably out of its 

 direction by the other, but the point of intersection is concealed by the road. The substance of 

 these dikes is so much decomposed that it would be impossible to recognise them for basalt except 

 by their resemblance to what occurs in other instances, where the rock is partly fresh and partly 

 decomposed ; indeed one of the dikes is changed into a substance very like a dun-coloured friable 

 sandstone. Besides these two dikes there are two others in the same situation, but they do not 

 present anything requiring particular notice. 



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