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In setting out from Brohl by the stream of the same name, which runs down from the Lake of Laach, where I was struck with the pieces of pumice-stone, and the charred remains of herbs and stalks of trees scattered over the marshes. I soon came to the valley, the sides of which are composed of what is called, in the language of geology, tufa, and in that of the country, dukstein, or trass.
The tuff consists of white felspathic mud, with fragments of slate and lava, reaching a depth in some places of 150 feet. After it has been quarried it is ground in mills, and used for cement stone under the name of trass.
Forests might be torn up by such a flood, and thus the occurrence of the numerous trunks of trees dispersed irregularly through the trass can be explained. The manner in which this trass conforms to the shape of the present valleys implies its comparatively modern origin, probably not dating farther back than the Pliocene Period. Volcanic Rocks of the Upper Miocene Period. Madeira. Grand Canary.
Its base consists 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, as is probable, this trass was formed during the period of volcanic eruptions, it may have originated in the manner of the moya of the Andes.
It shows that when some of these volcanoes were in action the valleys had already been eroded to their present depth. The tufaceous alluvium called trass, which has covered large areas in the Eifel, and choked up some valleys now partially re-excavated, is unstratified.
But the largest and most celebrated of all the sinter terraces has within the last few years been buried from view beneath a flood of volcanic trass, or mud, an event which was as unexpected as it was unwelcome.
You must not suppose the stream to be clear like the Aar, for it is as thick as pea-soup, and about the same colour, being in fact a river of trass in solution. You may almost fancy yourself on magic ground, and looking on a fairy castle, so peculiar is the effect.
While I am on the subject of volcanic phenomena, I may as well add a word on the origin of the trass or tufa, which is so thickly spread over this country.
I next reached Burgbrohl and Wassenach, passing several of the trass mills, for the stone is in many places hard enough for mill-stones, and there is a considerable trade in them to Holland, and thence to England and other countries.
The Brühl Valley, which unites with that of the Rhine at the town of that name, and drains the northern side of the volcanic region, has always been regarded with much interest by travellers for the presence of a deposit of "trass" with which it is partially filled. The origin of this valley was pre-volcanic, as it is hewn out of the slaty rocks of the district.
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