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



I shall first describe the several formations in detail, in the following; 

 order: Grauwacke, Trachyte, Trachyte TutF, Basalt, Brown-Coal For- 

 mation, Loess ; and shall then point out their several relations to each other, 

 drawing- such conclusions as to their several ages, and the geological history 

 of the district, as the facts appear to me to warrant. 



Graua^acke. 



This rock varies considerably in mineral character, sometimes occurring as 

 a fine-grained clay slate, not distinguishable from that of Westmoreland ; in 

 others, as a coarse red sandstone, very like some beds of the old red sandstone 

 of Herefordshire and Shropshire. Although I found no opportunities of 

 forming a decided opinion on the subject, it is probable that the prevailing- 

 rock in the neighbourhood of the Siebengebirge belongs to the later periods of 

 the grauwacke deposit. At Dottendorf, on the left bank of the Rhine, about 

 two miles from Bonn, there are beds of it of a loose texture containing 

 rounded pebbles of slate, with numerous vegetable impressions, which, in spe- 

 cimens, might be taken for a sandstone of the coal-nieasures. These impres- 

 sions are too indistinct in form to enable one to pronounce upon their nature ; 

 they are black, and when exposed to heat the blackness disappears. Impres- 

 sions of Terebratulae and the stalks of Encrinites, although abundant in the 

 grauwacke of the adjoining country, are not very common within this district. 

 They are found in that near Rhondorf, and in the neighbourhood of Unkel. 

 In some places it contains anthracite ; and I was informed by Mr. A. L. Sack 

 that at Bruckhausen six different beds of it were found, one of which was a 

 foot thick, but they were not continuous. 



The strata are generally highly inclined, but they are found at all angles. 

 There is neither uniformity in the strike nor in the dip, but the former is 

 most generally from north-east to south-west, and the dip more frequently 

 south than north. In the immediate neighbourhood of the Siebengebirge the 

 strata are thrown up in all directions, which, at first sight, might be thought 

 to be caused by the eruption of the volcanic matter; but there are great de- 

 rangements of the grauwacke in the Eifel and Westerwald, at a distance from 

 igneous rocks. Sudden variations in the strike and dip, the latter from ver- 

 tical to horizontal, (if I may so speak of dip,) may be well observed between 

 Unkel quarry and Remagen, and on the opposite side of the Rhine between 

 Erpel and Orsberg. 



The extent occupied by the grauwacke will be seen by an inspection of the accompanying Map. 

 It flanks the volcanic region of the Siebengebirge on the east and south, and appears in detached 

 spots on its western side, but it is wholly wanting on the north. On the left bank of the river it 

 constitutes the ridge from Remagen to Rolandswerth, but it is partially covered by the brown-coal 



