Geological Society. 221 



to the later ages of that deposit, and to approach in character, in 

 some parts of the district, to the old red sandstone. There are 

 no associated beds of limestone. The strata are in general highly 

 inclined, but they are found at all angles; and there is neither uni- 

 formity in the strike nor in the dip ; the strike is most usually N.E. 

 and S.W., the dip more frequently S. than N. In the immediate 

 neighbourhood of the Siebengebirge, the strata are thrown up in all 

 directions, evidently by the eruption of the volcanic matter. 



The whole of the later secondary series is wanting, and the 

 grauwacke strata are covered, unconformably, by a tertiary deposit, 

 consisting of beds of sand, sandstone, clays, and lignites, which col- 

 lectively constitute a brown coal formation. This is covered by an 

 extensive bed of gravel, and above the gravel is a loosely coherent, 

 sandy loam, containing land shells of existing species, and called 

 in the Rhine valley, Loess. From under the grauwacke there have 

 burst forth a variety of unstratified rocks, consisting of various 

 modifications of trachyte, trachytic tufF, basalt, and other modifi- 

 cations of trap. The main body of the Siebengebirge is composed 

 of these volcanic rocks. 



There are many varieties of the trachyte, from a highly crystalline 

 rock, with separate crystals of felspar of great size, very like a large- 

 grained granite, to a compact stone of uniform structure, like com- 

 pact felspar or phonolite. The trachytic tuff also assumes various 

 appearances, from that of a coarse conglomerate to a white earthy 

 substance, scarcely distinguishable from chalk. There is no evi- 

 dence of the trachyte having flowed in a stream, and the author saw 

 it only in one place in the form of a dyke. There are several 

 varieties of trap, but the most common is a compact black basalt, 

 in many places in perfect columns. There are numerous dykes 

 of basalt. A remarkable eruption of trap tuff, penetrated by ba- 

 saltic dykes, occurs at Siegburg, where three ernes, of about 200 feet 

 in height, rise abruptly in the midst of an alluvial plain, nearly on a 

 level with the Rhine. 



The author points out the affinity which Von Buch has shown to 

 exist between the mineral composition of all the unstratified rocks, 

 and how a series of insensible gradations could be formed, through 

 trachyte and the trap family? from granite to modern lava. He shows 

 that a suite of specimens could be collected in the Siebengebirge, 

 passing insensibly from large-grained white trachyte to compact black 

 basalt ) and that these hills afford many interesting facts corroborative 

 of the opinion advanced by M. Gustave Rose respecting the identity 

 of hornblende and augite. Notwithstanding, however, this connexion 

 between the several volcanic rocks, the author points out distinct 

 evidence of different epochs of formation among them. He is of opi- 

 nion that the greater part of the trachytic tuff was the first ejected; 

 that it was similar to those showers of scoriae and ashes which fre- 

 quently precede the eruptions of streams of lava ; and that it is not, 

 as some previous writers have supposed, a rock recomposed from the 

 disintegration of pre-existing trachyte. He saw the trachytic tuff 

 traversed in one place by a dyke of trachyte, and it contains numerous 



