THE NATURE AND USE OF FOSSILS 89 



time to time come into operation. The observer should, 

 however, be on his guard against the possibility that the 

 .submergence of tree-roots may sometimes not prove 

 - subsidence of land or rise of sea, or any change 

 of level at all Where, for instance, an inlet or creek 

 has been separated from the sea by a barrier of sand 

 or other detritus, land plants and even trees may grow 

 below the level of high-water. If the barrier is cut down 

 so as to admit the sea, the tides will rise over submerged 

 terrestrial vegetation, while all the time the relative levels 

 of sea and land may have remained unchanged. 



the tludrd ra hAk and ckys. 



Among the geological formations which form the 

 - part of the earth's crust, it is sometimes possible 

 to obtain . e sections wherein successive terres- 



trial movements and conditions of physical geography are 

 well illustrated A good example occurs in Joppa Quarry, 

 near Edinburgh. It will be seen from the accompanying 

 section (Fig. 2 1 ) that five seams of coal occur, each repre- 

 senting a terrestrial surface, or at least an aquatic floor 

 whereon grew a vegetation with its roots in the water and 

 its branches in the air. There must have been a pro- 

 gressive subsidence until the first formed coal-seam had 

 been buried under many feet of sand and mud which 

 inclosed also the remains of other similar terrestrial sur- 



