330 THE LOESS IN BELGIUM. CHAP. xvi. 



occupying positions often independent of the present lines of 

 drainage. To restore in imagination the geographical outline 

 of Picardy, to which rivers charged with so much homogeneous 

 loam, and running at such heights, may once have belonged, 

 is now impossible. 



In the valley of the Ehine, as I before observed, the main 

 body of the loess, instead of having been formed at succes 

 sively lower and lower levels as in the case of the basin of the 

 Somme, was deposited in a wide and deep preexisting basin, 

 or strath, bounded by lofty mountain chains, such as the Black 

 Forest, Vosges, and Odenwald. In some places the loam 

 accumulated to such a depth as first to fill the valley and 

 then to spread over the adjoining table-lands, as in the case 

 of the Lower Eifel, where it encircled some of the modern 

 volcanic cones of loose pumice and ashes. In these in 

 stances it does not appear to me that the volcanoes were in 

 eruption during the time of the deposition of the loess, as 

 some geologists have supposed. The interstrtaification of 

 loam and volcanic ejectamenta was probably occasioned by 

 the fluviatile mud having gradually enveloped the cones of 

 loose scoriae after they were completely formed. I am the 

 more inclined to embrace this view after having seen the 

 junction of granite and loess on the steep slopes of some of 

 the mountains bounding the great plain of the Ehine on its 

 right bank in the Berg-strasse. Thus between Darmstadt 

 and Heidelberg perpendicular sections are seen of loess 200 

 feet thick, at various heights above the river, some of them 

 at elevations of 800 feet and upwards. In one of these may 

 be seen, resting on the hill side of Melibocus in the Odenwald, 

 the usual yellow loam free from pebbles at its contact with a 

 steep slope of granite, but divided into horizontal layers for a 

 short distance from the line of junction. In these layers, 

 which abut against the granite, a mixture of mica and of 

 unrounded grains of quartz and felspar occur, evidently 



