Mr. Horner on the Geology of the Environs of Bonn. 437 



formation and by loess between these places ; and from Rolandseck northward, it is covered by the 

 brown-coal formation, except in a few places on the eastern side of the elevated plateau that runs 

 from Rolandseck in a north-west direction; as at Muffendorf, Friesdorf, Dottendorf, and Kessenich 

 in all which places it appears in highly inclined strata. 



Trachyte. 



There are several varieties of this rock. Glassy felspar is the chief ingre- 

 dient in all, mixed with hornblende, occasionally augite, quartz, magnetic 

 iron, and sometimes, but rarely, mica. Distinct crystals of glassy felspar, 

 often of great size, are imbedded in the base of most of the varieties. The 

 different aspects which the rock presents depend mainly on the felspar of the 

 base being largely crystalline or small granular, upon the proportion of horn- 

 blende and size of its crystals, upon the size and proportion of the separate 

 imbedded crystals of felspar and hornblende, and upon the colours imparted 

 by the different states of the iron which enters into the composition of all. 

 Some varieties are also produced by the felspar being decomposed. 



The places where the trachyte can be studied to most advantage are on the Drachenfels, the Wol- 

 kenburg, and the Stenzelberg, especially the two latter, where there are extensive quarries. The 

 trachyte of the Drachenfels is one of the most remarkable varieties : it is a gray stone, with a cry- 

 stalline base of felspar, and minute crystals of hornblende, including separate crystals of felspar, 

 often more than two inches long and an inch and a half in diameter. It has been for ages used as a 

 building-stone, and is upon the whole very durable, except that the separate crystals of felspar are 

 apt to fall out. The Cathedral of Cologne is built of it. The trachyte of the Stenzelberg contains 

 more hornblende, and has few imbedded crystals of felspar ; but those of hornblende often occur, 

 and are sometimes of a great size. In some places there are portions of the rock, separated by sharp 

 lines from the rest of the mass, of a dark fine-grained texture, so as to look like included fragments 

 of basalt ; but it is probable that they are fragments of slate, altered by the heat of the melted tra- 

 chyte, as there are other similar portions about whose origin there can be no doubt, the slaty struc- 

 ture being quite distinct. In the quarry of the Stenzelberg a face of rock about sixty feet in height 

 is exposed: it is divided by vertical rents into large tabular masses of irregular forms, and in some 

 places they are rounded, standing out like the stems of gigantic trees, the rock separating into 

 curved layers parallel to the axis, a tendency to exfoliation which is often observable in small 

 specimens. This appearance has been particularly described by Professor Noggerath*, and Pro- 

 fessor Goldfuss, in his Geognostiche Tafeln, has given a print representing the face of the quarry 

 at that time : it is now a good deal altered, and the appearance of exfoliation on the great scale is 

 at present not so marked as it is in the print. The trachyte in the upper part of the quarry is vesi- 

 cular, and it resembles the lava stream which is quarried for millstones at Nieder Mendig near the 

 Laacher See ; so much so, as to suggest the idea that the latter owes its origin to a melted trachyte. 

 Mr. Lyell, who visited this place with me, remarked the great resemblance of the trachyte to the 

 rock quarried above the Plateau de I'Aigle, not far from the baths of the Mont Dor in Auvergne. 

 The trachyte of the Wolkenburg resembles that of the Stenzelberg, including, although more rarely, 

 large separate crystals of hornblende, and occasionally large masses of quartz. It often contams 



* Das Gebirge in Rheinland-Westphalen, iv. 359. 

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