The trachyte-conglomerate--which seems to be associated with the upper beds of the brown coal strata--is traversed by dykes of trachyte of later date; and though it is difficult to trace the line between the two varieties of this rock on the ground, Dr. von Rath has recognised the general distinction between them, which consists in the greater abundance of hornblende and mica in the trachyte of the Wolkenburg than in that of the Drachenfels. The trachyte of the Drachenfels was probably the neck of a volcano which burst through the fundamental schists of the Devonian period. It is remarkable for the large crystals of sanidine (glassy felspar) which it contains, and has a rude columnar structure. The absence of any clearly-defined craters of eruption, such as are to be found in the Eifel district and on the left bank of the Rhine--as, for example, in the case of the Roderberg--may be regarded as sufficient evidence that this range is of comparatively high antiquity. It seems to bear the same relation to the more modern craters of the Eifel and Moselle that the Mont Dore and Cantal volcanoes do to those of the Puy de Dôme. In both cases, denudation carried on throughout perhaps the Pliocene and Post-Pliocene periods down to the present day has had the effect of demolishing the original craters; so that what we now observe as forming these ranges are the consolidated columns of original molten matter which filled the throats of the old volcanoes, or the sheets of lava which were extruded from them, but are now probably much reduced in size and extent. Having thus given a description of the older volcanic range on the right bank of the Rhine, we shall cross the river in search of some details regarding the more recent group of Rhenish volcanoes, commencing with that of the Roderberg, a remarkable hill a few miles south of Bonn, from which the view of the Seven Mountains was taken. [Illustration: Fig. 22.--Section of the extinct crater of the Roderberg on the bank of the Rhine, above Bonn.--(Original.)] (_d._) _The Roderberg._--This crater, which was visited by the author in 1857, is about one-fourth of a mile in diameter, and is in the form of a cup with gentle slopes on all sides. In its centre is a farmhouse surrounded by corn-fields. The general section through the hill is represented above (Fig. 22).