Mr. Horner on the Geology of the Environs of Bonn. 435 



much assistance in the course of my inquiry, not only in conversation, but 

 also by the liberality with which they laid open to me the valuable col- 

 lections in the museum of the University under their charge. I am further 

 much indebted to the kindness of the Count von Reust, President of the 

 Council of Mines in the Prussian Rhenish Provinces, who made the exten- 

 sive geological library and the documents belonging- to that establishment 

 freely accessible to me. 



General Structure. 



The lowest and the prevailing sedimentary deposit of the district is "-rau- 

 wacke, both in the form of sandstone and of slate, a continuation of the same 

 formation which covers so great an extent of country in the Lower Rhine and 

 in the Eifel; but here no limestone beds are associated with it. None of the 

 later secondary strata occur, and the grauwacke is covered unconformably by 

 a deposit of the tertiary period, consisting of a series of beds of sand, sand- 

 stones, clays, and lignites, which collectively constitute a brown-coal forma- 

 tion. This is covered by an extensive bed of gravel, and above the gravel is 

 a loosely coherent sandy loam^ containing terrestrial and aquatic shells of ex- 

 isting species, called in the Rhine valley by the technical name of Loess. From 

 under the grauwacke a variety of unstratified rocks have been erupted, con- 

 fsisting of various kinds of trachyte and trachyte tutf, basalt, dolerite, and 

 other modifications of trap ; and in one place volcanic scoriae. The main 

 body of the Siebengebirge is composed of these unstratified rocks. 



The structure of the Siebengebirge was long considered to be different in 

 its character from that of any other group in Germany ; and it is so with 

 the exception of two analogous formations, the one in Nassau, near Monta- 

 baur, lately described by Stifft*; the other not far from Gratz in Styria, of 

 which an account was published by M. von Buchf in 1820 ; and by Dr. Dau- 

 beny in 1826 1. It is remarkable that the trachyte of Nassau had not been 

 previously examined by geologists : it is accompanied, as in the Siebengebirge, 

 by trachyte tuff and phonolite, and rises in the midst of a basaltic district of 

 vast extent, connected v»'ith that of the Westerwald. Professor Mitscherlich 

 visited that part of Nassau in 1832, and I saw him just after he had been 

 there. He described the trachyte as covering a space quite as great in extent 

 as the Siebengebirge, but not forming such elevated hills, the trachyte out- 

 bursts rarely exceeding a very moderate elevation above the surface. In 

 texture they very much resemble those of the Siebengebirge, and, like them, 

 present various modifications of the constituent parts. 



* Geognostiche Beschreihung des Herzogthums Nassau, 1831, 

 -j- Ueber einige Berge der Trap Formation bei Grdtz. 

 J Description of Active and Extinct Volcanos. 

 VOL. IV. SECOND SERIES. 3 L 



