112 Mr Lyell o?i the Loainy Deposit 



as in the loess, the drift-shells belong chiefly to terrestrial spe- 

 cies, and in both the great mass of the shells are referable to the 

 same genera, the principal difference consisting in the absence 

 from the loess of species of the genera neritina, ancylus, and 

 unio. The only bivalve-shells I ever happened to meet with in 

 the loess, were Cyclas fontinalis, Drap *. 



It may be well to observe here, that, in some places where the 

 bank of the river is wholly or partly composed of loess, the fossil 

 shells are often washed out, and may be found entire on the 

 shore ; and they might, in such cases, unless great caution were 

 used, be confounded with the more modern shells drifted down 

 by the Rhine. I was careful to guard against this source of 

 error, by collecting chiefly from spots far from the loess, and by 

 rejecting those which, by their want of colour, or by the circum- 

 stance of their being filled with loess, resembled the general cha- 

 racters of the fossils. The colour of the more modern specimens 

 affords in general a safe criterion for distinguishing them from 

 the fossils ; and I feel sure that there was scarcely any inter- 

 mixture in the sets above compared, or only two or three lym- 

 nea, at least, were doubtful. 



The greater part of the shells drifted by the Rhine agree spe- 

 cifically with those which are buried in the loess ; and if I had 

 enlarged my collection, the correspondence would no doubt have 

 been much more perfect, for the shells of the loess vary in differ- 

 ent localities, and those now brought down by the Rhine probably 

 vary equally at different seasons. As the drift shells of the 

 Rhine agree with those of the loess, so the sediment of that river 

 bears a very close resemblance to loess. This was first pointed 

 out to me by Professor Noegerath, and it has lately been con- 

 firmed by Mr Horner's experiments on the quantity and nature 

 of the solid matter brought down in the waters of the Rhine at 

 • 

 • I found several specimens of this with both valves entire, together with 

 Valvata piscinalis, in the interior of an individual of the Lymnea ovata, in 

 loess at Odenau, near Bruchsal. Hard calcareous concretions, in the same 

 loess, contained shells of recent helix and clausilia, which were thus embed- 

 ded in solid limestone. In the third volume of my Principles of Geology, 

 Appendix, p. 58, 1 included Cyclas palustris, and C. lacustris, Draj). in a list of 

 loess shells ; but I afterwards ascertained that they had been brought to the 

 spot in mud used to fertilize the soil. Probably they are to be found in 

 loess. 



