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. It is similar to that found near Naples, at Mont d'Or,
Carbal, and other parts of Italy; and, indeed, all the products of the
latter district are pretty nearly the same as these, allowing for the
difference of a slate surface in the one case, and a sandy and alluvial
soil in the other. The idea of the trass having any connexion with a
deluge, is, I believe, now exploded; and geologists have agreed that it
is the actual substance ejected by the volcano, subsided into a firm
paste. The rain has always been observed to fall heavily after
eruptions, and the water running down the sides of the hills, has formed
this crust, which makes the bottom and sides of the Laach. The same
causes are in action now; and if ever the lake should rise so high as to
burst its banks, it would overflow the whole country, and carry terrible
destruction with it. Such an event was actually foreseen by the
sagacious monks who formerly inhabited the abbey, for they cut a canal
nearly a mile long, to give the water vent; and the discharge by it
continues to this day.
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