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On the opposite side of the crater, overlooking the Rhine, we find the cliff of Rolandsec composed of hard vesicular lava, rudely prismatic, and extending from the summit of the hill to its base, about 250 feet below. This is the most northerly of the group of the Eifel volcanoes.
The electrometer gave no sign of electricity. As the storm gathered, the blue of the sky changed at first to deep azure and then to grey. The vesicular vapour became visible, and the thermometer rose three degrees, as is almost always the case, within the tropics, from a cloudy sky which reflects the radiant heat of the soil. A heavy rain fell.
These angular pseudo fragments consist of the first-described dark green rock, of a brown softer kind, of serpentine, and of a yellowish harsh stone, which, perhaps, is related to serpentine rock. There are other vesicular, calcareo-ferruginous, soft stones.
Sandstone overlying coal; trap dykes; syenitic porphyry dykes; black vesicular trap, penetrating in thin veins the clay shale of the country, converting it into porcellanite, and partially crystallizing the coal. On this sandstone lie fossil palms, and coniferous trees converted into silica, as on a similar rock in Angola. IGNEOUS ROCKS. Trappean rocks, with hot fountain.
About 100 yards higher up the stream black vesicular trap is seen, penetrating in thin veins the clay shale of the country, converting it into porcellanite, and partially crystallizing the coal with which it came into contact.
The material of which the Puy de Dôme is formed consists of a light grey, nearly white, soft felsitic lava, containing crystals of mica, hornblende, and specular iron-ore. It is highly vesicular, and was probably extruded in a pasty condition from a throat piercing the granitic plateau and the overlying sheet of ancient lava of Mont Dore.
At the east side of the lake we find a stream of scoriaceous lava of a purple or reddish colour, highly vesicular, and containing crystals of mica; but the most important lava-stream is that which has taken a southerly direction from the crater of the Laacher See towards Nieder Mendig and Mayen, for a distance of about six miles.
Before the cone was built, a flood of rough vesicular lava was poured into the lake, cutting it in two, and, overflowing its banks, the fiery flood advanced into the pine-woods, overwhelming the trees in its way, the charred ends of some of which may still be seen projecting from beneath the snout of the lava-stream where it came to rest.
Every here and there, though, there were hot sandy patches dotted with peculiar-looking black stone lying in masses, cracked and riven as if by fire, while parts were cindery and vesicular, others glistening in the sunshine like black glass. "You take the lead now, Carey," said the doctor. "You can't go wrong; only follow the river; it will lead you right up to the lake."
The formation of this group is a little singular, the calcareous limestone on Deal occurring two hundred feet above the sea and between granite; whilst on Erith vesicular lava was found.
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