FOSSIL SHELLS OF THE LOESS.

Figures 44, 45, and 46

  (FIGURE 44. Succinea oblonga.)

  (FIGURE 45. Pupa muscorum.)

  (FIGURE 46. Helix hispida, Lin.; H. plebeia, Drap.)

The loess is generally devoid of fossils, although in many places they are abundant, consisting of land-shells, all of living species, and comprising no small part of the entire molluscous fauna now inhabiting the same region. The three shells most frequently met with are those represented in the annexed figures (44, 45 and 46). The slug, called Succinea, is not strictly 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 Limnaea, Planorbis, Paludina, Cyclas, and others, requiring to be constantly in the water, are extremely exceptional in the loess, occurring only at the bottom of the deposit where it begins to alternate with ancient river-gravel on which it usually reposes.

This underlying gravel consists in the valley of the Rhine 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 northwards 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 neighbourhood of the principal mountain chains bordering the great valley, such as the Black Forest, Vosges, and Odenwald, an admixture 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.