Formations on the Left Bank of' the Rhine. 155 



partly owing to the sulphuric or muriatic acids it contains,) 

 and the evaporating effect of this current on the wet floor of 

 the grotto from which it issues, which is moistened by some 

 superficial rill, will be sufficient to coat it with ice in summer 

 since the more rarified by heat the external air, the more ra- 

 pid will be the current of cool dry air, and, consequently, 

 the evaporation. In winter, a similar draught of air, though 

 less rapid, will be produced ; and taking the temperature of 

 the rocks through which it passes, which, from the depth of 

 the gallery, will be about the mean annual temperature of that 

 climate, must appear warm compared with the external air, to 

 the shepherds who seek a shelter at the mouth of the fissure. 

 The cone of Roth connects itself with a smaller ridgy hill 

 prolonged towards the Kyll, which has given rise to three or 

 four small distinct streams of basaltic lava. 



On approaching the Kyll towards Gerolstein, the traveller 

 is struck by the appearance of an elevated plateau formed of 

 Jura limestone in horizontal strata, resting on the quader- 

 sandstone, and bounded by a range of picturesque and cragoy 

 cliffs, with a talus of massive debris at their base. From the 

 surface of the plateau rise four large volcanic cones, besides 

 small eminences of a similar nature. One has given rise to a 

 current of basalt, which descends the steep cliffs of limestone 

 in a sort of cascade, on the western side, occupies a small bot- 

 tom, and, winding round the base of the range of rocks, 

 reaches the channel of the Kyll at Sarsdorf. * 



The two largest cones of this plateau, lie N. W. of Cassel- 

 burg, a romantic ruin of great picturesque beauty about two 

 miles N. of Gerolstein. 



Round Rockeskill, there are traces of another aqueous for- 

 mation of peperino similar to that of Steffler, and appearing to 

 have originated in the hill immediately behind that village. 

 Further north, the Waldsdorfer Kopf is a very regular cone, 

 and at its foot lies a crater-basin, once a lake, but now re- 

 duced to a peat-moss. The cone has emitted one of the 

 largest currents of lava of this district. It has flowed towards 

 the west, and reaches nearly to Hillesheim. 



Arnsberg is a large and complete cone, which has also pro- 



* A sketch of this interesting fact is giving at the bottom of Plate IV. 



