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



to be mentioned ; the numerous fragments of it included in the volcanic rocks, 

 and the basalt rocks traversing it, remove all doubt. The great disturbance 

 of the grauwacke strata in the neighbourhood of the trachyte and basalt may 

 have been produced by their eruption. 



It is clear that volcanic eruptions took place during the deposition of the beds 

 of the brown-coal formation. 



Trachyte tuff covers the sandstone at Quegstein, alternates with clay and 

 clay ironstone in the neighbourhood of Rott and Utweiler, contains fragments 

 of wood at the Langenberg, and leaves of trees, identical with those found in 

 the papierkohle and sandstones, at Scheuern and Ofenkulenberg*. The basalt 

 tuff of Siegburg contains fragments of wood, bituminized and penetrated by 

 iron, not to be distinguished from that found in the lignite deposits. In con- 

 formity with what has been said of the more modern age of the basalt, we 

 find it at Utweiler lying over the brown-coal beds to the depth of thirty feet; 

 and no fragments of basalt occur in any of the beds of the brown-coal forma- 

 tion, as far as I have been able to ascertain. Besides the instance of alternate 

 beds mentioned above, the trachyte tuff in many places shows an arrangement 

 that could only have been produced by the materials being moved under the 

 surface of water. 



All these facts seem to me to prove that volcanic eruptions were going on 

 in a freshwater lake, in the same manner as we have submarine eruptions at 

 the present day, during the time that the brown-coal beds were in the course 

 of being deposited. It is probable, however, that all the volcanic rocks we 

 now see were not ejected at that period, but that subsequent eruptions took 

 place which heaved up the Siebengebirge and the cones to the south, carrying 

 up the brown-coal beds along with them, in some instances, as at Stosschen, 

 to the height of nine hundred feet above the present surface of the Rhine; 

 the same action, probably, heaving up the plateau on the left bank of the river, 

 although with a less degree of force, as the basaltic outbursts on that side are 

 comparatively on a limited scale. The great fault in the brown-coal beds, 

 described p. 459, indicates a powerful and sudden disturbing force. It would 

 appear, also, that the great mass of gravel which covers the brown-coal beds 

 had been strewed over them previous to this elevation, for it is found on both 

 sides of the Rhine at a great height, and never in the intermediate plain, the 

 gravel of which, as far as my observations go, has quite a distinct character. 



I have endeavoured to obtain some distinct evidence as to the relative age 

 of the volcanic eruption of the Roderberg, described p. 447, the whole cha- 

 racters of which are very different from anything in the Siebengebirge. I 



* See Appendix X. p. 478. 



