278 THE WONDERS OF GEOLOGY. Lect. III. 



these beds the volcanic eruptions, consisting of trachyte, 

 basalt, and other modifications of trap rocks and scoriae, 

 have forced their way. The basalt is black, and very com- 

 pact, and breaks into shafp fragments ; it is frequently 

 columnar, and the separate hexagonal pillars are made use 

 of for posts and pavements, in the adjacent towns. Such, says 

 Mr. Horner,* is the profusion of basaltic pillars, that the 

 walls of the town of Linz are wholly built of these materials, 

 placed on their sides, with the ends projecting outwards. 

 The streets are paved with the smaller columns set on end, 

 thus forming a miniature representation of the Giants' 

 Causeway ; and the same volcanic product forms a large 

 proportion of the walls of Bonn and Cologne. The second- 

 ary strata are covered by a series of tertiary deposits, 

 consisting of sand, sandstone, clay, and lignite ; constituting 

 what is termed a brown coal formation. Upon these 

 deposits is an extensive layer of gravel, which is covered 

 by a deep bed of loosely coherent sandy loam, or mud, 

 provincially termed Loess, This fine earthy detritus, which 

 contains recent species of terrestrial and fresh- water shells, 

 forms the subsoil of the vast plains in which Bonn and 

 Coblentz are situated, and extends as far as the falls of 

 Schaffhausen. 



55. Brown coal formation, — As the usual condition 

 in which' bituminized vegetable matter occurs in the tertiary 

 formations, is well exemplified in the brown coal, or lignite, 

 of the Rhine, it will be instructive to examine the characters 

 of this deposit somewhat in detail ; for we shall thereby 

 obtain data which will prepare us for the investigation of 

 the ancient carboniferous system. This formation, which 

 is spread over a great extent of country on both sides of the 

 river, consists of clay, sand, sandstone, conglomerates, 

 clay and ironstone, with lignite, or bituminized wood, of 



* On the Geology of the Environs of Bonn, by Leonard Horner, 

 Esq. F.E.S. Geological Transactions, vol. iv. 1836. 



