48 THE STORY OF THE EARTH AND MAN. 



Annulosa. And it is most interesting thus to find in 

 these very old rocks the modern subdivisions of 

 animals already represented, and these by types some 

 of them nearly allied to existing inhabitants of the 

 seas. I have endeavoured in the engraving to repre- 

 sent some of the leading forms of marine life in this 

 ancient period. 



Perhaps one of the most interesting discoveries in 

 these rocks is that of rain-marks and shrinkage-cracks, 

 in some of the very oldest beds those of the Long- 

 mynd in Shropshire. On the modern muddy beach 

 any ordinary observer is familiar with the cracks 

 produced by the action of the sun and air on the dried 

 surfaces left by the tides. Such cracks, covered by 

 the waters of a succeeding tide, may be buried in 

 newer silt, and once preserved in this way are im- 

 perishable. In like manner, the pits left by passing 

 showers of rain on the mud recently left bare by the 

 tide may, when the mud has dried, become sufficiently 

 firm to be preserved. In this way we have rain-marks 

 of various geological ages; but the oldest known are 

 those of the Longmynd, where they are associated 

 both with ripple-marks and shrinkage-cracks. We 

 thus have evidence of the action of tides, of sun, and 

 of rain, in these ancient periods just as in the present 

 day. Were there no land animals to prowl along the 

 low tidal flats in search of food ? Were there no herbs 

 or trees to drink in the rains and flourish in the sun- 

 shine ? If there were, no bone or footprint on the 

 shore, or drifted leaf or branch, has yet revealed their 

 existence to the eyes of geologists. 



