Mr. Horner on the Geology of the Environs of Bonn. 443 



balls of trachyte which it contains are very often entirely different from any 

 variety that now^ exists en masse. In addition to all these objections to its 

 being derived from preexisting trachyte, I think there is every reason to sup- 

 pose that the ejection of the tuff preceded that of the solid trachyte, just as in 

 existing volcanos showers of ashes often precede streams of lava. 



Basalt. 



Although all the rocks I am about to describe under this head do not strictly 

 correspond in mineralogical character with the black compact substance to 

 which the term is usually applied, I shall not convey any erroneous idea by 

 employing it, as by far the greater proportion of the trap rocks of the district 

 are true basalt, characterized by the presence of olivine, and the rest have a 

 great resemblance to it. 



Basalt occurs in different parts of the district on both sides of the Rhine. In the Siebengebirge 

 it forms entire hills, and dikes of it traverse grauwacke, trachyte, and trachyte tuff; it is also as- 

 sociated with the beds of the brown coal formation. The three highest hills in the district are 

 composed of it, viz. the Diisberg near Linz, the Oehlberg, and the Lowenburg in the Siebengebirge ; 

 and, by a reference to the Table of Heights in the Appendix, it will be seen that it constitutes one 

 half of the separate hills in the group. The extent of their united surfaces is also fully equal to 

 that occupied by trachyte. 



There are two important differences as regards the manner in which the 

 two classes of rocks have been ejected : the trachyte, except in a single in- 

 stance, is not found in veins traversing other rocks, and the basalt is never 

 associated with basaltic tuff except at Siegburg, to be afterwards described, 

 and that hill is not within the Siebengebirge group. 



The summit of the Lowenburg is composed of a rock that would be called 

 dolerite by most geologists, its constituent parts, augite and felspar, being in 

 much larger grains than in basalt : it contains magnetic pyrites, and it sepa- 

 rates in some places into slender irregular columns. 



The upper part of the Oehlberg is a close-grained black basalt in large tabular or rudely columnar 

 masses, highly magnetic, and possessing polarity ; it contains in some places numerous reddish 

 brown grains, which are probably hyacinth, as that mineral is not unfrequently met with in the 

 basalt of the district : among the specimens which accompany this paper is one from Papelsberg 

 near Ober-Cassel, with a well-defined crystal of hyacinth*. The ridge between Ober-Cassel and 

 Ramersdorf is wholly composed of basalt, containing much common calcareous spar and arrago- 

 nite, but no zeolite, a substance which seems to be of very rare occurrence in the Siebengebirge. 

 Numerous quarries have been opened in this ridge ; in the most considerable of them the rock has 



* Sassenberg, the intelligent guide of the Siebengebirge at Ober-Dollendorf, told me in July 1833 

 that a specimen of basalt had been found in the quarry at Unkel containing sapphire. 

 VOL. IV. SECOND SERIES. 3 M 



