438 Mr. Horner on the Geologi/ of the Environs of Bonn. 



calcareous spar, lining cavities, and in well-defined crystals. Here, also, a lofty face of rock is ex- 

 posed in the quarry, exhibiting the trachyte in large vertical tabular masses, without apparent cross 

 joints, although these exist, as the rock separates easily into huge blocks by natural fissures. It is 

 for the most part of a light fawn colour ; but when tlie large masses are broken they show a bluish 

 gray nucleus, such as is often seen in trap rocks. The one colour, however, does not always gra- 

 duate into the other ; on the contrary, tliere is sometimes a sharp line between them, and at the 

 separation the trachyte is of a vitreous texture. I observed the same thing in the trachyte of the 

 Keilsbrunnen quarry, with a tendency to globular decomposition. One of the most remarkable 

 varieties of trachyte is that of the Perlen Hardt, on the eastern side of the group. I nowhere found 

 such large imbedded crystals as here, but they are extremely brittle, and consequently good spe- 

 cimens are not easily obtained*. The base is much decomposed, the felspar being in some places 

 changed into a bright red earth : it contains small yellow crystals of sphene. At the Rosenau there 

 is a variety containing augite ; I obtained a specimen at this place with a separate well-defined 

 crystal of that mineral ; and close to this spot I found a sudden change into a variety with large 

 imbedded crystals of felspar, like that of the Drachenfels, a circumstance which, like many others, 

 shows under how many different forms the same materials may issue from a volcanic focus. At 

 the Klein Rosenau there occurs a variety of a light gray colour, very compact, with a conchoidal 

 fracture and ringing sound, a phonolite, composed of felspar and quartz, without any hornblende 

 or other ingredient ; and it passes into another kind of a light fawn colour, still more compact, in 

 which the quartz predominates almost to the exclusion of the felspar, being infusible, while the 

 gray variety is readily acted upon by the blowpipe : the latter is in some places of a bright pale 

 green, apparently from an admixture of iron, and contains chalcedony, hyalite, and veins of ferru- 

 ginous opalf. 



The Hummerich, the Mittelberg, and the Bruder-Kunzberg are three conical hills, which rise 

 out of an elevated plateau of grauwacke at some distance to the south of the main group of the 

 Siebengebirge. They are composed of trachyte, but of a kind bearing a strong resemblance to 

 some varieties of trap, particularly that of the Mittelberg and Bruder-Kunzberg. It is of a gray 

 colour, has a much smaller proportion of distinct crystals of felspar and hornblende, and has some- 

 what of a foliaceous structure. At the Hummerich it is rudely columnar, and has a tendency to 

 globular decomposition : at the foot of the same hill it has the character of an amygdaloid. 



The affinity between trachyte and the trap family has been pointed out by 

 many geologists ; long- ago by M. von Buch in his admirable essay on Trap Por- 

 phyry;};. In the Siebengebirge it would be easy to form a suite of specimens^ 

 showing a gradation from a white trachyte to a compact black basalt. The 

 analogy is further seen by the columnar form and tendency to globular de- 



* Lavas, which probably belong to a much more modern period than these rocks, contain fel- 

 spar crystals of great size. In his description of the Ponza Isles, Mr. Poulett Scrope says, " I met 

 " with some felspar crystals nearly three inches in diameter, and these are accompanied by augite, 

 " bearing the same proportion to them in size as the ordinary minute grains to the common-sized 

 " felspar."- — Geological Transactions, Second Series, vol. ii. p. 215. 



f Mr. Poulett Scrope in the same Memoir, describes a siliceous trachyte, which appears from 

 the desci'iption to be very similar to the rock found at the Klein Rosenau. — Geological Trans- 

 actions, Second Series, vol. ii. pp. 20\, 208, 214, 229. 



J Transactions of the Royal Academy of Berlin, 1812. 



