TERTIAKY VOLCANIC ROCKS. [Ch. XXXI. 



di Bolsena, and others in Italy, being surrounded by a ridge of gently 

 sloping bills, composed of loose tuffs, scoriae, and blocks of a variety 

 of lavas. • 



One of the most interesting volcanoes on the left bank of the Ehine 

 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 over- 

 spread by quartzose gravel, and this again is covered by volcanic 

 scoriae and tufaceous sand. The opposite wall of the crater is com- 

 posed of cinders and scorified rock, like that at the summit of Vesu- 

 vius. 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 the loess. 



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 

 sandstone 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 volca- 

 noes more worthy of note, than the proofs they afford of very copi- 

 ous aeriform discharges, unaccompanied by the pouring out of melted 

 matter, except, here and there, in very insignificant volume. I know 

 of no other extinct volcanoes where gaseous explosions of such mag- 

 nitude 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 lifted 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 Chapter 

 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 reexcavated, 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 numerous trunks and branches of trees. If, as is probable, this 

 trass was formed during the period of volcanic eruptions, it may have 

 originated in the manner of the moya of the Andes. 



