Formations on the Left Bank of the Rhine. ] 49 



scribed above, as occurring in the conglomerate, crystals of 

 lazulite, &c. 



The volcano of Laach appears, like so many in other coun- 

 tries, to have produced at first trachytic, and afterwards ba- 

 saltic lavas. The trass and the pumice conglomerates, which 

 chiefly compose its surrounding eminences, belong decidedly 

 to the former class of products ; and, though trachyte, as a 

 rock, does not, I believe, show itself in situ, it probably ex- 

 ists, concealed by the fragmentary strata of the hills, or the 

 thick woods which cover their slopes, and render it difficult to 

 examine their composition. The origin of the trass has been 

 variously accounted for, and sometimes ascribed to deluges 

 and other similar hypothetical events. It appears to me to de- 

 rive simply from an ordinary modification of the volcanic phe- 

 nomena. The pulverulent matter, of which it was principally 

 composed, mixes into a retentive paste or clay with water, so 

 indeed, as to be used for making pottery, where it is found 

 in a loose state. In this state it was ejected by the volcano, 

 and thrown up as usual into a circular or elliptical ridge 

 around the orifice. The rain, which falls generally in great 

 abundance after the termination of an eruption, mixed with 

 these trachytic ashes, must often have formed an impermeable 

 crust at the bottom, and upon the sides of this cavity. Hence 

 the water that drains down these slopes would accumulate in- 

 to a lake continually increasing in depth, until either the 

 pressure of its waters breaks down the banks on some one 

 side, or a fresh eruption from below displaces it. In either 

 case, a breach being made in the circumference of the crater, 

 the contents |of the lake must rush out in a violent debacle, 

 carrying off great quantities of the fragmentary matter of the 

 hills through which the water bursts, and filling with these allu- 

 vial deposits the vallies by which it escapes on the plains at 

 the foot of the volcano. 



This process may be many times repeated from the same 

 volcanic orifice, and, I think, is without doubt the real histo- 

 ry of the tufas of the left bank of the Rhine, as well as of 

 those of the Mont-Dor, Cantal, and of some parts of Italy. 

 Whether the mass hardened afterwards, or remained incoherent, 

 appears to have depended chiefly on the qualitv of the ashes, 



