In the Rhineland of Germany, between Mainz and Bonn, there are a number of basalt mountains and formations. The area to the west of the Rhine, known as the Westerwald, and the region to the east, known as the Eifel, is dotted with eroded volcanic cones and outcrops of basalt. Some of the cities, such as Linz, are largely built of basalt that has been quarried from the surrounding countryside. Near the village of Unkel, the basalt extends under the riverbed, and until they were removed, basalt columns used to stick up above the river surface, presenting a considerable impediment to navigation. The volcanic nature of the Rhineland basalt was not recognized until the last quarter of the eighteenth century.
The illustration to this section is a detail of the plate in exhibit item 30, and shows the basalt columns in the Rhine near Unkel.