534 
ELEMENTS OF GEOLOGY. 
series of volcanic products might be detected of every age 
from the Older Pliocene to the historical epoch. 
Pliocene Volcanoes of the Eifel ,—Some of the most perfect 
cones and craters in Europe, not even excepting those of the 
district round Vesuvius, may be seen on the left or west 
bank of the Rhine, near Bonn and Andernach. They exhibit 
characters distinct from any which I have observed else¬ 
where, owing to the large part which the escape of aqueous 
vapor has played in the eruptions and the small quantities 
of lava emitted. The fundamental rocks of the district are 
gray and red sandstones and shales, with some associated 
limestones, replete with fossils of the Devonian or Old Red 
Sandstone group. The volcanoes broke out in the midst of 
these inclined strata, and when the present systems of hills 
and valleys had already been formed. The eruptions occur¬ 
red sometimes at the bottom of deep valleys, sometimes on 
the summit of hills, and frequently on intervening platforms. 
In travelling through this district we often come upon them 
most unexpectedly, and may find ourselves on the very edge 
of a crater before we had been led to suspect that we were 
approaching the site of any igneous outburst. Thus, for ex¬ 
ample, on arriving at the village of Gemund, immediately 
south of Daun, we leave the stream, which flows at the bot¬ 
tom of a deep valley in which strata of sandstone and shale 
crop out. We then climb a steep hill, on the surface of 
which we see the edges of the same strata dipping inward 
towards the mountain. When we have ascended to a con¬ 
siderable height, we see fragments of scoriaB sparingly scat¬ 
tered over the surface; until at length, on reaching the sum¬ 
mit, we find ourselves suddenly on the edge of a tan\ or deep 
circular lake-basin called the Gemunder Maar. In it we rec¬ 
ognize the ordinary form of a crater, for which we have 
been prepared by the occurrence of scorige scattered over the 
surface of the soil. But on examining the walls of the crater 
W’e find precipices of sandstone and shale which exhibit no 
signs of the action of heat; and we look in vain for those 
beds of lava and scoriae, dipping outward on every side, 
which we have been accustomed to consider as characteris¬ 
tic of volcanic vents. As we proceed, however, to the oppo¬ 
site side of the lake, we find a considerable quantity of scoriae 
and some lava, and see the whole surface of the soil spark¬ 
ling with volcanic sand, and strewed with ejected fragments 
of half-fused shale, which preserves its laminated texture in 
the interior, while it has a vitrified or scoriform coating. 
Other crater lakes of circular or oval form, and hollowed 
out of similar ancient strata, occur in the Upper Eifel, where 
