330 THE LOESS IN BELGIUM. chap. xvi. 



and 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 pre-existing 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 

 instances 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 suj)posed. The interstratification of 

 loam and volcanic ejectamenta was probably occasioned by 

 the fluviatile mud having gradually enveloped the cones of 

 loose scorise 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 Rhine on its 

 right bank in the Berg-strasse. Thus, between Darmstadt 

 and Heidelberg iDcrpendicular 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 Oden- 

 wald, 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 occurs, evidently 



