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 Roderherg. — One of the most interesting vol- 
canos on the left bank of the Rhine is called the Roderbero". 
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 graywacke 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 scorise 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 
