EIFEL.^TRASS. 
535 
copious aeriform discharges have taken place,.throwing out 
vast heaps of pulverized shale into the air. I know of no 
other extinct volcanoes 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 lifted up the stratified rocks immediately around the 
vent so as to form conical masses, having their strata dip¬ 
ping outward on all sides from a central axis, as is assumed 
in the theory of elevation craters, alluded to in the last 
chapter. 
I have already given (p. 512, Fig. 590) an example in the 
Eifel of a small stream of lava which issued from one of the 
craters of that district at Bertrich-Baden. It shows that 
when some of these volcanoes were in action the valleys had 
already been eroded to their present depth. 
Trass .—The tufaceous alluvium called trass.^ which has 
covered large areas in the Eifel, 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 numerous trunks and branches of trees. If, as 
is probable, this trass was formed -during the period of vol¬ 
canic eruptions, it may have originated in the manner of the 
moya of the Andes. 
We may easily conceive that a similar mass might now be 
produced, if a copious evolution of gases should occur in one 
of the lake-basins. If a breach should be made in the side 
of the cone, the flood would sweep away great heaps of eject¬ 
ed fragments of shale and sandstone, which would be borne 
down into the adjoining valleys. Forests might be torn up 
by such a flood, and thus the occurrence of the numerous 
trunks of trees dispersed irregularly through the trass can 
be explained. The manner in which this trass conforms to 
the shape of the present valleys implies its comparatively 
modern origin, probably not dating farther back than the 
Pliocene Period. 
