Ch. XIV.] 
AGE OF EIFEL VOLCANOS. 
199 
covered the graywacke ; and basaltic currents of a somewhat 
later date have followed. 
There is, however, such a connexion between these rocks 
that a suite might be procured from the Siebengebirge, 
showing an insensible gradation from highly crystalline tra- 
chyte into compact basalt, with the accompanying passage of 
the hornblende in the former, into augite in the latter. 
Age of the volcanic rocks of the Eifel uncertain. — Besides 
the ancient inclined graywacke, we have in the immediate 
vicinity of the valley of the Rhine, a nearly horizontal tertiary 
formation, called brown coal, from the association with it of 
beds of lignite worked for coal. The great mass of the igneous 
rocks are seen to be newer than this formation ; and thus we 
obtain a relative date of much local importance for the volcanos 
of the whole region. This brown coal consists of beds of sand 
and sandstone, with nodules of clay-ironstone, and siliceous 
conglomerate. Beds of lignite of various thickness are inter- 
stratified with the clays and sands, and often irregularly dif- 
fused through them. This deposit was classed with the plastic 
clay at a time when every group of tertiary strata was referred 
to the age of some one of the subdivisions of the Paris basin, 
but as no shells, either marine, fresh-water, or land have 
yet been found imbedded, it is not easy to decide the age 
of the formation. Near Marienforst, in the vicinity of Bonn, 
large blocks are found on the surface of a white opaque 
quartz rock, containing numerous casts of fresh-water shells 
which appear to belong to Planorbis rotundatus and Limnea 
longiscatus, two well-known Eocene species * ; but this rock is 
not in situ, and may possibly have been a local deposit in some 
small lake, fed by a spring holding silica in solution. Yet, as 
there are beds of the brown coal at Marienforst, and this for- 
mation contains in other places subordinate beds of silex, it 
seems to me most probable that the quartzose blocks alluded 
to were derived from some member of that tertiary group. 
* M. Deshayes, to whom I showed the specimens, said he felt as confident of 
the above identifications as mere casts would warrant, 
