Ch. X.] LOESS OF THE RHINE. 123 



the pumiceous superstratum ; and this last follows the slope of the hill, 

 just as it would have done had it fallen in showers from the air on a 

 declivity partly formed of loess. 



But, in general, the loess overlies all the volcanic products, even those 

 between Neuwied and Bonn, which have the most modern aspect ; and 

 it has filled up in part the crater of the Roderberg, an extinct vc»lcano 

 near Bonn. In 1833 a well was sunk at the bottom of this crater, 

 through 70 feet of loess, in part of which were the usual calcareous con- 

 cretions. 



The interstratification above alluded to, of loess with layers of pumice 

 and volcanic ashes, has led to the opinion that both during and since its 

 deposition some of the last volcanic eruptions of the Lower Eifel have 

 taken place. Should such a conclusion be adopted, we should be called 

 upon to assign a very modern date to these eruptions. This curious 

 point, therefore, deserves to be reconsidered ; since it may possibly have 

 happened that the waters of the Rhine, swollen by the melting of snow 

 and ice, and flowing at a great height through a valley choked up with 

 loess, may have swept away the loose superficial scoriae and pumice of 

 the Eifel volcanoes, and spread them out occasionally over the yellow 

 loam. Sometimes, also, the melting of snow on the slope of small vol- 

 canic cones may have given rise to local floods, capable of sweeping down 

 light pumice into the adjacent low grounds. 



The first idea which has occurred to most geologists, after examining 

 the loess between Mayence and Basle, is to imagine that a great lake 

 once extended throughout the valley of the Rhine between those two 

 places. Such a lake may have sent oft' large branches up the course of 

 the Main, ISTeckar, and other tributary valleys, in all of which large 

 patches of loess are now seen. The barrier of the lake might be placed 

 somewhere in the narrow and picturesque gorge of the Rhine between 

 Bingen and Bonn. But this theory fails altogether to explain the phe- 

 nomena ; when we discover that that gorge itself has once been filled 

 with loess, which must have been tranquilly deposited in it, as also in 

 the lateral valley of the Lahn, communicating with the gorge. The 

 loess has also overspread the high adjoining platform near the village of 

 Plaidt above Andernach. Nay, on proceeding farther down to the north, 

 we discover that the hills which skirt the great valley between Bonn and 

 Cologne have loess on their flanks, which also covers here and there the 

 gravel of the plain as far as Cologne, and the nearest rising grounds. 



Besides these objections to the lake theory, the loess is met with near 

 Basle, capping hills more than 1200 feet above the sea ; so that a barrier 

 of land capable of separating the supposed lake from the ocean would re- 

 quire to be, at least, as high as the mountains called the Siebengebirge, 

 near Bonn, the loftiest summit of which, the Oehlberg, is 1209 feet above 

 the Rhine, and 1369 above the sea. It would be necessary, moreover, to 

 place this lofty barrier somewhere below Cologne, or precisely where the 

 level of the land is now lowest. 



Instead, therefore, of supposing one continuous lake of sufiicient extent 



