ON ITS EXTERIOR. VOLCANOES. 235 



perhaps near Urmits, on the left side of the Ehine. 

 From the great friability of this substance, the later 

 action of the river may have entirely removed all traces 

 of its place of origin or eruption. In the whole line of 

 the Eifel Maars, as well as in the Eifel volcanoes, from 

 Bertrich to Ormond, no pumice is found. That of the 

 Laacher See is confined to the hills of its shores, and at 

 the other Maars the small pieces of felspar, lying in the 

 volcanic sand and tufa, do not pass into pumice." 



We have already alluded to the relations which the 

 age of the Maars, and that of the very different eruptions 

 of lava, bear to the age of the formation of the valleys. 

 " The trachyte of the Siebengebirge seems much older 

 than the formation of the valleys, and even than the 

 Ehenish browncoal. Its origin is unconnected with 

 the rending open of the Ehine valley, even supposing 

 we should ascribe the formation of that valley to the 

 opening of a fissure. The valley is much more recent 

 in its origin than the Rhenish browncoal, and more 

 recent than most of the Ehenish basalt; on the other 

 hand, it is older than the volcanic eruptions of lava- 

 streams, and older than the great outburst of pumice, 

 and than the trass. Basaltic formations decidedly come 

 down to a more recent period than do those of trachyte ; 

 and the principal mass of the basalt is thus to be looked 

 upon as younger than the trachyte. On the present 

 sides of the valley of the Ehine many basaltic groups 

 (those of Unkeler Steinbruch, Eolandseck, and Godes- 

 berg) were, it may be supposed, first laid bare by the 

 opening of the valley, as it is probable that they were 

 previously enclosed in the Devonian graywacke hills." 



The Infusoria (whose general distribution over our 



