36 ANCIENT PLANTS 



and debris under an ocean, and after being formed have 

 seen daylight on a land surface long ago, and sunk again 

 to be covered by newer deposits, perhaps even a second 

 or a third time, before they rose for the time that is the 

 present. Yet all these profound changes took place so 

 slowly that had we been living then we could have felt 

 no motion, just as we feel no motion to-day, though the 

 land is continuing to change all around us. The great 

 alternations between land and water over large areas 

 mark out to some extent the main periods tabulated on 

 p. 34, for after each great submersion the rising land 

 seems to have harboured plants and animals with some- 

 what different characters from those which inhabited it 

 before. Similarly, when the next submersion laid down 

 more rocks of limestone and sandstone, they enclosed the 

 shells of some creatures different from those which had 

 inhabited the seas of the region previously. 



Through all the periods the actual rocks formed 

 are very similar shales, limestones, sandstones, clays. 

 When any rocks happen to have preserved neither 

 plant nor animal remains it is almost impossible to tell 

 to which epoch they belong, except from a comparative 

 study of their position as regards other rocks which do 

 retain fossils. This depends on the fact that the phy- 

 sical processes of rock building have gone on throughout 

 the history of the globe on very much the same lines 

 as they are following at present. By the sifting power 

 of water, fine mud, sand, pebbles, and other debris are 

 separated from each other and collected in masses like 

 to like. The fine mud will harden into shales, sand- 

 grains massed together harden into sandstones, and so 

 on, and when, after being raised once more to form dry 

 land, they are broken up by wind and rain and brought 

 down again to the sea, they settle out once again in a 

 similar way and form new shales and sandstones; and so 

 on indefinitely. But meantime the living things, both 

 plant and animal, have been changing, growing, evolv- 

 ing, and the leafy twig brought down with the sand- 



