Mr. Horner on the Geology of the Environs of Bonn. 467 



Of the relative Ages of the Volcanic Rocks to each other and to the Sedimen- 

 tary Rocks. 



The trachyte tuff appears to me to be the oldest of the volcanic eruptions,, 

 for it has nothing of the character of having been derived from preexistino- 

 trachytes, and it is traversed by a dike of trachyte, as I have described at 

 p. 439. This is quite in accordance M^ith what takes place in active volcanos, 

 where nothing is more common than for an eruption of ashes to precede that 

 of a stream of lava ; and it is also in accordance with what has taken place in 

 another part of this district, at Siegburg, where dikes of basalt traverse basaltic 

 tuff. The masses of solid trachyte contained in the tuff are not fragments, 

 but, as was observed to me by Professor Mitscherlich while we were ex- 

 amining together the trachyte tuff of the Konigswinter Holle, are similar to the 

 bombs we had found in such abundance in the ashes of the volcanos of the 

 Eifel, and which are of frequent occurrence in the ashes of active volcanos*. 

 They often, it is true, resemble, nay, are almost identical with, the trachyte of 

 the adjoining hills, but they are very frequently of a kind unlike any found en 

 masse. That there may be some secondary trachyte conglomerates is very 

 probable ; indeed I am inclined to consider that variety at Ober Cassel, con- 

 taining portions of basalt and balls of clay ironstone, as having been derived 

 from preexisting trachyte and trachyte tuff. 



Previous writers on this district have said that they were unable to point 

 out any difference of age between the trachyte and the basalt. I presume, 

 therefore, that the dike of amygdaloidal basalt which traverses the trachyte in 

 the Keilsbrunnen quarry must have been only lately exposed, for that is de- 

 cisive of the question. Another proof of the basalt being more modern than 

 the trachyte is, that the trachyte tuff, except in the case of that at Ober Cassel 

 above alluded to, never, as far as my observations and information go, con- 

 tains any detached masses of basalt. There is, besides, no instance of a tra- 

 chyte dike traversing basalt, but many of the latter traversing trachyte tuff. 



That the trachyte and basalt are later than the grauwacke requires scarcely 



* Mr. Poulett Scrope in describing the trachyte tuff of the Ponza Isles (which he calls conglo- 

 merate,) mentions the occurrence of numerous " fragmentary blocks " in it, and says, " None of the 

 fragments are water-worn, and they all have evidently been enveloped in the base immediately upon 

 their ejection by the volcanic impetus." He also shows that eruptions of solid trachyte have taken 

 place subsequently to the ejection of the tuff: "this rock [prismatic trachyte] bears on most points 

 the appearance of having forced its way upwards by cutting through the incumbent conglomerate 

 in various directions, and occasionally of having spread laterally over it." — Geological TransaC' 

 tions, Second Series, ii. 203 and 206. 



VOL. IV. — SECOND SERIES. 3 P 



