HISTORY FROM THE ROCKS 191 



and renewal of land observed in the present has been going on 

 throughout geological time ; and that the changes the earth 

 has gone through have taken up very many millions of years. 



Biologically, we find that the further we go back in time the 

 more unlike were the animals and plants to those living to-day ; 

 that they have steadily progressed towards existing types ; that the 

 progress was from simpler to more complex and more highly special- 

 ised forms ; that the progress was along evolutionary lines ; and 

 that there has been a constant relationship between progress in 

 the kingdoms of life and their environment. 



Fossils should be collected from any rocks which contain 

 them. They will usually be obtained better from weathered 

 rocks and spoil-banks than from the rock exposure itself. As 

 might be expected from the fact that the sea is the great repository 

 of sediment, the majority of stratified rocks are of marine origin, 

 and they will show this by the types to which their fossils belong. 

 It may be possible to ascertain by comparison with modern genera, 

 not only that the organisms were marine, but the approximate 

 depth or conditions under which they lived ; whether between tide 

 marks, in shallow water, in deep water; or whether they were 

 free-swimming and liable to be deposited at all depths. They 

 may also give an indication of the climatal conditions under which 

 they lived. It must be remembered that only those with hard 

 skeletons or shells are likely to survive in the fossil state, and it 

 may be shown that even then their preservation is generally 

 due to chemical replacement or alteration of their substance. 

 Sometimes we have only external moulds or casts of the exterior 

 of hollow bodies to judge by, and there may be cases where only 

 certain structures, like the nacre of shells or the vascular tissue 

 of plants, have been preserved. 



The fossils of other strata may prove that they were laid down 

 in lakes or by rivers, in deltas or even on land areas. In the 

 first two cases we shall have only the remains of fresh-water 

 animals and plants, pond snails, river mussels, and the like, with 

 a certain number of land forms that have been washed in. In 

 deltas similar remains will be mingled with brackish water and 

 marine forms. Land deposits will sometimes consist of vegetable 

 agglomerations, like peat or coal and lignite seams, or of barren 



