CHAP. XVI. FOSSIL SHELLS OF THE LOESS. 327 



the annexed figures. The slug culled Succinea is not stvictlj'' 

 aquatic, but lives in damp places, and may be seen in full 

 activity far from, rivers, in meadows, where the grass is wet 

 with rain or dew; but shells of the genera Limnea, Planorlns, 

 Paludina, Cyclas, and others, requiring to be constantly in the 

 water, are extremely exceptional in the loess, occurring onlj- 

 at the bottom of the dejjosit, where it begins to alternate with 

 ancient river-gravel, on which it usually reposes. 



This underlying gravel consists, in the valley of the Ehine, 



Fis. 44. Fis. 45. Fig. 46. 



Succinea clonijata. Pupa musconcm. Helix hisjyida Lin. ; H. jilebeitmi Jeffrey?. 



for the most part, of pebbles and boulders of Alpine origin, 

 showing that there was a time when the rivers had power to 

 convey coarse materials for hundreds of miles northward 

 from Switzerland, towards the sea; whereas, at a later period, 

 an entire change was brought about in the physical geography 

 of the same district, so that the same river deposited nothing 

 but fine mud, which accumulated to a thickness of 800 feet 

 or more above the original alluvial plain. 



But, although most of the fundamental gravel was derived 

 from the Alps, there has been observed in the neighborhood 

 of the principal mountain-chains bordering the great valley, 

 such as the Black Forest, Yosges, and Odenwald, an ad- 

 mixture of detritus characteristic of those several chains. 

 We cannot doubt, therefore, that as some of these mountains, 

 especially the Vosges, had, during the glacial period, their 

 own glaciers, a part of the fine mud of their moraines must 

 have been mingled with loess of Alpine origin ; although the 

 principal mass of the latter must have come from Switzerland, 

 and can in fact be traced continuously from Basle to Belgium. 



22 



