CaXXXL] MIOCENE VOLCANIC ROCKS. 543 



lava, as they fell round the vent, were cemented together into one com- 

 pact mass, in consequence of continuing to be in a half-melted state. 



If we pass from the upper to the lower Eifel, from a to b (see map, p. 

 538), we find the celebrated lake-crater of Laach, which has a greater re- 

 semblance than any of those before mentioned to the Lago di Bolsena, 

 and others in Italy — being surrounded by a ridge of gently slopiug hills, 

 composed of loose tuffs, scoriae, and blocks of a variety of lavas. 



One of the most interesting volcanos on the left bank of the Rhine, near 

 Bonn, is called the Roderberg. It forms a circular crater nearly a quarter 

 of a mile in diameter, and 100 feet deep, now covered with fields of corn. 

 The highly inclined strata of ancient sandstone and shale rise even to 

 the rim of one side of the crater ; but they are overspread by quartzose 

 gravel, and this again is covered by volcanic scoriae and tufaceous sand. 

 The opposite wall of the crater is composed of cinders and scorified 

 rock, like that at the summit of Vesuvius. It is quite evident that the 

 eruption in this case burst through the sandstone and alluvium which 

 immediately overlies it; and I observed some of the quartz pebbles 

 mixed with scoriae on the flanks of the mountain, as if they had been 

 cast up into the air, and had fallen again with the volcanic ashes. I 

 have already observed, that a large part of this crater has been filled up 

 with loess (p. 123). 



The most striking peculiarity of a great many of the craters above 

 described, is the absence of any signs of alteration or torrefaction in 

 their walls, when these are composed of regular strata of ancient sand- 

 stone and shale. It is evident that the summits of hills formed of the 

 above-mentioned stratified rocks have, in some cases, been carried away 

 by gaseous explosions, while at the same time no lava, and often a very 

 small quantity only of scoriae, has escaped from the newly-formed cavity. 

 There is, indeed, no feature in the Eifel volcanos more worthy of note, 

 than the proofs they afford of very copious aeriform discharges, unac- 

 companied by the pouring out of melted matter, except, here and there, 

 in very insignificant volume. I know of no other extinct volcanos 

 where gaseous explosions of such magnitude have been attended by the 

 emission of so small a quantity of lava. Yet I looked in vain in the 

 Eifel for any appearances which could lend support to the hypothesis, 

 that the sudden rushing out of such enormous volumes of gas had ever 

 'ifted up the stratified rocks immediately around the vent, so as to form 

 conical masses, having their strata dipping outwards on all sides from a 

 central axis, as is assumed in the theory of elevation craters, alluded to 

 in Chap. XXIX, 



Trass. — In the Lower Eifel, eruptions of trachytic lava preceded the 

 emission of currents of basalt, and immense quantities of pumice were 

 thrown out wherever trachyte issued. The tufaceous alluvium called 

 trass, which has covered large areas in this region and choked up some 

 valleys now partially re-excavated, is unstratified. 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 nume- 



