318 MARKINGS, FOOTPRINTS AND FUCOIDS 



also that these casts in relief are the best preserved. This 

 arises from the fact that the original moulds or impressions 

 are usually made in clay, whereas the filling material is sandy, 

 and the latter, infiltrated with calcareous or siliceous matter, 

 may become a hard sandstone, while the clay may remain a 

 comparatively soft shale. This tendency of casts rather than 

 of moulds to be preserved sometimes produces puzzling effects. 

 A cylindrical or branching trail thus often assumes the appear- 

 ance of a stem, and any pits or marginal impressions assume 

 the form of projections or leaves, and thus a trail of a worm 

 or Gastropod or a rill mark may easily simulate a plant. It is 

 to be observed, however, that these prominent casts are on 

 the under side of the beds, that their material is continuous 

 with that of the beds to which they belong, and that they are 

 destitute of any carbonaceous matter. There are, however, 

 cases where markings may be in relief, even on the upper 

 surfaces of beds. The following are illustrations of this. Just 

 as a man walking in newly fallen snow compresses it under his 

 feet, and if the snow be afterwards drifted away or melted 

 away by the sun, the compressed part resists longest, and may 

 appear as a raised footmark, so tracks made on soft material 

 may consolidate it so that if the soft mud be afterwards washed 

 away the tracks may remain projecting. Again, worms eject 

 earthy matter from their burrows, forming mounds, patches or 

 raised ridges of various forms on the surface, and some animals 

 burrow immediately under the surface, pushing up the mud 

 over them into a ridge, while others pile up over their bodies 

 pellets of clay, forming an archway or tunnel as they go. 

 Zeiller has shown that the mole cricket forms curious roofed 

 trails of this kind, and it seems certain that Crustaceans and 

 marine worms of different kinds execute similar works, and 

 that their roofed burrows, either entire or fallen in, produce 

 curious imitations of branches of plants. 



The great and multiform army of the sea worms is indeed 



