Mr. Horner on the Geology of the Environs of Bonn. 439 



composition. Near the Thier Garten, north of the Stenzelberg, there is a 

 variety of trachyte scarcely distinguishable from the base of an earthy amyg- 

 daloid ; and I found a block at the foot of the Drachenfels, of a green colour, 

 very like some kinds of serpentine. Nor is this intimate connexion between 

 these two classes of unstratified rocks different from what might naturally have 

 been expected to happen, for the materials of both exist in the same spot; 

 variations in the proportions of these not very numerous ingredients, in the 

 degrees of heat to which they were exposed, in the circumstances under 

 which they cooled, would perhaps account for all the shades of difference 

 we meet with. M. Von Buch states in the memoir above quoted, that 

 wlien trachyte passes into a black substance, the felspar and hornblende 

 have disappeared, and olivine and augite have taken their places. Much 

 light has been thrown on this subject by the interesting experiments of 

 Mr. G. Rose*, who has shown how intimately hornblende and augite are 

 connected : he obtained artificial crystals of the forms of augite by fusing 

 hornblende ; and he conceives that hornblende may be formed by slow, 

 and augite by rapid cooling. Such gradations show how necessary it is to be 

 cautious in drawing geological inferences from mere mineralogical variations, 

 and that in giving new names to rocks merely on account of such differences, 

 much confusion and error may be introduced. Notwithstanding, however, 

 this connexion between the trachyte and basalt of this district, the basalt ap- 

 pears to have been, if not entirely, at least in great part, erupted at a distinct 

 and later period, and I have not discovered any evidence of trachyte having 

 been again ejected after the basalt had begun to flowf. 



There is only one place where I have seen the trachyte in the form of a 

 dike or vein, and I have not been able to learn that it is so met with in any 

 other spot. This dike crosses the road from the Lowenburg Tranke to the 

 Perlen Hardt. It is about five feet wide, and is much decomposed, but the 

 fresher parts resemble the trachyte of the Perlen Hardt. It traverses a tra- 

 chyte tuff, containing hard masses of trachyte and fragments of slate. 



* Leonhard and Bronn's Jahrhuch filr Mineralogie, &c., 1832, p. 237. 



t Among the volcanic rocks of the Siebengebirge are varieties whose constituent parts are as 

 largely crystalline as any granites that have been met with. Von Buch states that in several places 

 in the neighbourhood of Clermont and Puy de Dome, a transition from granite into trachyte may 

 be traced, and thus we have the gradation extended from granite to basalt. Dr. Hibbert de- 

 scribes a gradual transition from basalt into granite in the Shetland Islands. (Brewster's Edinburgh 

 Journal of Science, i. 107.) Speaking with Professor Mitscherlich about the large imbedded 

 crystals of the Drachenfels and Perlen Hardt, he remarked to me, that in a mass which is only 

 viscid, crystals will be formed, but that it requires a very long time ; that felspar becomes viscid 

 at a comparatively moderate heat, but that it requires a very high temperature to make it flow. 



