MARKINGS, FOOTPRINTS AND FUCOIDS 317 



has time to consolidate and become dried by the sun and air 

 before the next tide, much better impressions are preserved ; 

 and lastly, on those parts of the shore which are reached only 

 by the spring tides, the mud of the highest tide of course may 

 have several days to harden before the next tide reaches it, 

 and in this case it becomes cracked by an infinity of shrinkage 

 cracks, which, when it is next covered with the tide, are filled 

 with new sediment. In this way is produced in great perfec- 

 tion that combination of footprints, or even of impressions of 

 rain, with casts of cracks, which is so often seen in the older 

 rocks. Where on the sides of channels or near the shore'the 

 mud has a considerable slope, another and very curious effect 

 results. As the tide ebbs the water drains off the surface, or 

 oozing out of the wet sand and mud, forms at the top of the 

 bank minute grooves often no larger than fine threads. These 

 coalesce and form small channels, and these, again, larger ones, 

 till at low tide the whole sloping surface is seen to be covered 

 with a smooth and beautiful tracery resembling the rivers on 

 a map, or the impressions of the trunks and branches of trees, 

 or the fronds of gigantic seaweeds. These " rill marks," as 

 they have been called, are found in great abundance in the 

 coal formation and triassic sandstones and shales, and I am 

 sorry to say, have often been named and described as Fucoids, 

 and illustrated by sumptuous plates. Sometimes these im- 

 pressions are so fine as to resemble the venation of leaves, 

 sometimes so large as to simulate trees, and I have even seen 

 them complicated with shrinkage cracks, the edges of which 

 were minutely crenulated by little rills running into them from 

 the surface. 



It is further to be noticed that all these markings and im- 

 pressions on tidal shores may, when covered by succeeding 

 deposits, appear either in intaglio or relief. On the upper 

 surface they are of course sunken, but on the lower surface of 

 the bed deposited on them they are in relief. It often happens 



