Formation* on the Left Bank of the Rhine: J 47 



rate, or lapillo, containing numerous pumice stones, fragments 

 of a phonolitic lava of clay slate, partly calcined, &c. 



Thin beds of these fragmentary matters also occasionally 

 cover the flat parts of the slate plateau in the vicinity of the 

 cones, or occupy a few bosoming hollows in the slopes of its 

 valleys. 



Many of these vallies are also filled to a considerable height, 

 often to more than half their total depth, with indurated 

 tufa, called in the dialect of the country Dukstein or Trass, 

 of which an immense quantity is quarried on numerous points, 

 and carried down the Rhine into Holland, where it is in great 

 request for buildings. The lower part of the mass is univer- 

 sally the most solid and compact, and hence is preferred by 

 the quarrymen. It passes gradually into loose arenaceous tu- 

 fa towards the upper part of the deposit. This tufa re- 

 sembles extremely that of Naples (particularly of Capo di 

 Monte and Posilipo). When freshly quarried, it is thorough- 

 ly saturated with water, which is driven out by every blow of 

 a hammer upon it. In this state it is of a dull bluish black 

 colour, but, on drying, it assumes a shade of light grey. It 

 appears to be almost wholly composed of fragmentary pumice, 

 and is evidently a conglomerate. It contains also frao-ments of 

 a slaty or phonolitic, and of amorphous basalt, of burnt clay- 

 slate, and a great quantity of carbonized wood, not in frag- 

 ments or beds, but consisting of whole trunks or branches, 

 which penetrate the rock in all directions. The condition of 

 this wood is very nearly that of common charcoal, but it pul- 

 verizes more readily, and often of its own accord, on exposure. 

 In the valley of Burg-bruhl the trass rests sometimes im- 

 mediately on the clay-slate, but, on other points, a bed of 

 calc-tuff intervenes, the deposit of some mineral spring prior 

 to the deposition of the tufa. A similar incrustation occa- 

 sionally overlies the trass, and has enveloped fragments of 

 pumice, forming a species of calcareous tufa. The indurat- 

 ed tufa is sometimes divided into massive beds by interven- 

 ing layers of loose pumice or lapillo, and fragmentary clay- 

 slate. 



On ascending the valley of the Bruhl, I found this trass 

 deposit occupying it to a great depth the whole way from its 



