54 THE STORY OF THE EARTH. 



as of those parts which are hidden under the oceans. 

 Their vertical succession depends upon successive 

 changes in position of the area from which sedi- 

 ments are brought into the sea. 



If the land sinks down so that it becomes 

 smaller, and its shores recede, each kind of sedi- 

 ment derived from it, being carried by the moving 

 water the same distance as before its depression, 

 is transported for a less distance out to sea as 

 compared with the deposit formed previously. 

 Therefore the finer sediments on a sinking sea- 

 bed rest upon the coarser sediments, which had 

 been formed previously, when the source of sup- 

 ply was nearer to the place of deposition. In 

 other words, clays rest upon sands; while the new 

 sands rest upon areas which had previously been 

 dry land. If this process of depression continues, 

 then while the clays follow the new sands, and 

 become super-imposed upon them, limestones are 

 super-imposed upon clays. 



Sandstones occasionally give evidence that 

 they were deposited between tide marks, in pre- 

 serving the footprints of animals, as well as m 

 the ripple marks, sun-cracks and rain-prints which 

 Were formed when the surfaces dried between suc- 

 :ve tides. Such memorials are preserved in 

 the Trias Sandstone of Cheshire, and the Hast- 



a Sand. Clays occasionally, in the abundant 



remains of terrestrial plants which they yield, 

 give evidence of estuarine origin, which may not 



Strictly comparable to the succession of con- 

 ditions seen upon a land which is being Mib- 



.•ed. 



()n the other hand sonic deposits are formed 



upon shores whii h are rising, and advance at the 



expense Of the sea, and then the deposits which 



