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



the remarkable appearance of a gigantic globular concretion, an ellipsoid, the shorter axis of which 

 is at least seventy feet, composed of concentric layers. Professor Noggerath* has described this 

 spot ; and although by the operations of quarrying the appearances are a good deal altered since 

 his drawing was made, the concentric arrangement of the rock is quite distinct. In a quarry north 

 of this the basalt is split into tabular masses like stratification ; and these again into smaller portions 

 by cross rents, so that the face of the rock looks like a dry-built wall of squared stones, an appear- 

 ance not at all uncommon in modern lavas. In a quarry under Fovaux's Hauschen, a little further 

 north, the basalt presents the appearance of a pile of globular concretions so squeezed together as 

 to assume the form of flattened lenticular masses. 



In the southern part of the district the sharp cone which rises from the grauwacke plateau above 

 Honnef, called the Leyberg, is composed of rudely columnar basalt ; and between it and Linz are 

 four other conspicuous hills, rising in a like manner from the grauwacke ; the Gross Dusberg, 

 1325 feet above the Rhine, the Klein Diisberg, the Hummelskopf, and the Mendenberg, all com- 

 posed of basalt. On approaching the Mendenberg from Erpel or Linz, one is surprised to see 

 the quantity of fragments of the most regular slender basaltic columns, which are applied to all 

 sorts of purposes, as building-stones ; pig-styes are constructed of pillars which would be prized 

 as beautiful specimens in a public museum. The quarry from which these are procured is situated 

 immediately under the summit of the Mendenberg ; it forms at present an oval space of about 

 fifty feet by thirty, the entrance to which is by a narrow passage, and it is surrounded by walls 

 twenty feet high, of the most regular slender columns, standing nearly erect, and packed close 

 together like the cells of a honeycomb, but easily separable and without cross joints, so that single 

 columns are sometimes obtained of the whole height : I saw one lying on the ground, a hexagon 

 of nine inches diameter and sixteen feet long. The most regular columns are pentagons and hexa- 

 gons ; in these the angles are very sharp. It is a very compact black basalt, which breaks into 

 very sharp fragments that ring like glass, and contains few imbedded minerals : I obtained, how- 

 ever, a good specimen of slender prismatic mesotype. The ends of the columns reach the surface, 

 and are not covered by amorphous basalt ; but they are an inferior bed, for higher up, at the very 

 summit, there is a mass of columns less regular in form, lying in an inclined position, and detached 

 from each other. Such is the profusion of basaltic columns in this neighbourhood, that the walls 

 of the town of Linz are wholly built of them, laid on their sides, with the ends projecting outwards ; 

 and the streets are paved with the smaller columns set on end, a miniature representation of the 

 Giant's Causeway. The walls of Bonn and Cologne are in the same way built of fragments of 

 basaltic columns laid with brick ; and the posts by the sides of the roads and streets in this part 

 of the country are generally made of the same ready-shaped stones. 



All round Linz basalt breaks out from the grauwacke at detached points. One of the most con- 

 siderable of these eruptions is the Erpeler Lei, which rises to a very considerable height from the 

 edge of the river. These basalt knolls are of very frequent occurrence towards the east and south- 

 east of this district, and they form a continuation of the eruptions around Siegen, in the VVester- 

 wald and near Frankfurt ; and these are connected with the greater masses of the Vogelgebirge 

 Rhongebirge, and Habichtwald. It breaks out, too, at several detached points on the left bank 

 of the Rhine within this district. The Landskron in the lower part of the Ahr valley, a hill about 

 800 feet high, is composed of grauwacke for two thirds of its height ; but the upper part is basalt, 

 showing some tolerably regular jointed columns. Opposite to the town of Unkel a considerable 

 face of rock is exposed in an extensive quarry : here the whole mass of basalt is columnar, the 



* Rheinland-Westphalen, ii. 250. See also Dr. Daubeny on Volcanos, p. 67. 



