Mr. Horner on the Geology of the Environs of Bonn. 447 



be distinguished in specimens from the Rowley Rag of Staffordshire^ or the 

 Castle Rock of Edinburgh, 



The only remaining point of volcanic eruption which I have to notice is 

 the Roderberg, a hill above the village of Mehlem, on the left bank of 

 the Rhine. From a distance^ and even near at hand, one would not suspect 

 that the hill had anything volcanic in its nature ; it is not conical,, but looks 

 like a smooth rounded prolongation of the adjoining hills that belong to the 

 brown-coal formation. But at the beginning of the ascent from Mehlem 

 loose black cinders appear, and on arriving at the summit there is found a 

 circular depression of about a hundred feet deep, and a quarter of a mile in 

 diameter, once the crater of a volcano, but now occupied by corn-fields sur- 

 rounding a farm-house. It is on the north and north-east side chiefly that the 

 volcanic materials are seen, for in the ascent from Nieder Bachem there are 

 few traces of them, till one comes to the edge of the crater, even although the 

 road is cut several feet deep into the hill in some places. On the north and 

 north-east the edge of the crater is composed of a spongy scoriaceous rock, 

 with marks of fusion, identical with the productions of active volcanos. At 

 the south-west side of the crater, however, grauwacke appears, but whether m 

 situ or in detached blocks it is difficult to determine ; it is of that brighter red 

 colour from oxidation of iron which is produced when a portion of grauwacke 

 is exposed to a strong heat. Upon this lies a yellow gravel, the same as that 

 which covers the brown-coal formation throughout the district, and containing 

 no fragments of volcanic matter ; and upon the gravel lies a black tufaceous 

 semi-indurated substance, disposed in layers which slope outwards from the 

 crater*. 



Brown-Coal Formation. 

 This formation consists of an assemblage of beds of siliceous sand, sand- 

 stone, quartzose conglomerate, clay of different qualities, clay ironstone in 

 layers and detached masses in the clay, and of lignite of various qualities, in 

 distinct beds and intermixed with the clay. 



It occupies a great extent of country on both sides of the Rhine. On the right bank it covers 

 the northern slope of the Siebengebirge, as it falls to the plain through which the Sieg flows, oc- 

 cupying an area of about seven miles from east to west, and five from north to south. It occurs 

 again north of the Sieg, in detached spots along the sides of the hills forming the eastern boundary 

 of the Rhine valley, as far as the immediate vicinity of Bensberg, nine miles east of Cologne. On 

 the left bank of the river it forms a long broad ridge of low hills, or rather plateau, lying between 

 the valleys of the Rhine and the Erft, from the town of Meckenheim, nine miles south-west of Bonn, 



* See Appendix IV. p. 472. 



