198 OLDER PLIOCENE PERIOD. [Ch. XIV. 



the tufaceous alluvium of the Rhine volcanos called trass, 

 which has covered large areas, and choked up some valleys 

 now partially re-excavated. This trass is, like the loess, un- 

 stratified. The base is composed almost entirely of pumice, 

 in which are included fragments of basalt and other lavas, 

 pieces of burnt shale, slate, and sandstone, and numerous 

 trunks and branches of trees. 



If an eruption, attended by a copious evolution of gases, 

 should now happen in one of the lake basins, we might suppose 

 the water to remain for weeks in a state of violent ebullition, 

 until it became of the consistency of mud, just as the sea be- 

 came charged with red mud round the new island of Sciacca, 

 in the Mediterranean, in the year 1831. If a breach should 

 then be made in the side of the cone, the flood would sweep 

 away great heaps of ejected fragments of shale and sandstone, 

 which would be borne down into the adjoining valleys. 

 Forests would be torn up by such a flood, which would ex- 

 plain the occurrence of the numerous trunks of trees dispersed 

 irregularly through the trass. 



Crater of the Roderberg. One of the most interesting vol- 

 canos on the left bank of the Rhine is called the Roderberg. 

 It forms a circular crater nearly a quarter of a mile in diameter, 

 and one hundred feet deep, now covered with fields of corn. 

 The highly inclined gray wacke strata rise even to the rim of one 

 side of the crater, but they are overspread by quartzose gravel, 

 and this again is covered by volcanic scoriae and tufaceous 

 sand. The opposite wall of the crater is a scoriaceous rock, 

 like that at the summit of Vesuvius. It is quite evident that 

 the eruption in this case burst through the graywacke and 

 alluvium which immediately overlies it ; and I observed some 

 of the quartz pebbles mixed with scoriae on the flanks of the 

 mountain, so placed as if they had been cast up into the air, 

 and had fallen again with the volcanic ashes. 



On the opposite, or right bank of the Rhine, are the Sieben- 

 gebirge, a group of mountains wherein analogous phenomena 

 are exhibited. There also trachytic lavas have flowed out and 



