1 50 Mr Poulett Scrope's observations on the Volcanic 



and their intimate commixture with the water. This indura- 

 tion is evidently a chemical process, analogous to the setting 

 of cements and mortars. - The mud eruptions (tepetate) of 

 Quito, and the tufas of Iceland, are produced by the same 

 train of circumstances in the present day. As the filling up 

 of the crater must usually be a slow process, a sufficient in- 

 terval will often occur before the lake bursts through its sides, 

 either by its own weight, or the occurrence of an eruption, 

 for the slopes to be covered by vegetation, and even by whole 

 forests of trees, which, when the banks give way, will be 

 hurried along, and buried within the torrent of mud, (or li- 

 quid tufa,) where they are afterwards carbonized, probably 

 by long exposure to the moisture which penetrates the whole 

 rock.* 



With regard to the trass of Laach and its vicinity, this ex- 

 planation is peculiarly applicable ; and the lake would, even 

 at this day, be subject to rise until it burst its bank, but for 

 an artificial channel, or emissary, cut for its drainage by the 

 monks of the abbey of Laach, a picturesque ruin which stands 

 on its western side. Currents of tufa appear to have been 

 discharged in this manner from many points of the circum- 

 ference of the lake. Those that issued on the eastern side 

 occupied the vallies of the Brohl, and other streams which 

 empty themselves into the Rhine ; the remainder inundated 

 the slate plateau in the direction of Niedermennig, Bell, Ol- 

 burgh, and Kruft, and covered it more or less with beds of com- 

 pact tufa, which alternate with others of similar composition, 

 but loose and incoherent, probably deriving from the frag- 

 mentary ejections of the neighbouring vents. 



A cavern within the basin of the lake of Laach gives out a 

 considerable volume of carbonic acid gas, presenting all the 

 phenomena of the Grotta del Cane. There are also many mi- 

 neral springs in the vicinity, as at Tonigstein, and near the 



" Since we hear that numerous trees are found in a carbonized slate, 

 amongst the substances blown into the air by some of the paroxysmal ex- 

 plosions of the Javanese antl Polynesian volcanos, it remains doubtful 

 whether this character is always owing, as in this case, to torrefactionjby 

 volcanic heat, or occasionally to long maceration in water. Are these trees 

 only charred on their surfaces, or throughout, like the surturbrand ? 





