THE SUCCESSION OF STRATA. 53 



continuity of deposits of the open sea. These 

 breaks are local, like the breaks which are made 

 by the islands or lands which divide the sea, and 

 by the waters which separate lands from each 

 other. 



The Succession of super-imposed Rocks. 



A sediment may be followed round a shore 

 line, so that it has everywhere the same general 

 character, except in so far as the rocks of the 

 cliffs vary, which give rise to pebble beds in some 



ilities a s any sandy particles on the 



shore in others. As a rule, tidal work sorts and 



- the products which the sea carries down to 



depths, so that they are arranged in bands 

 which arc parallel to the coast. The particles 



vary in size in those zones of deposit. The finer 



particles remain suspended longest; and arc there- 

 fore transported to the greatest distance by the 



moving water. Thus there is a horizontal succes- 



of rocks "ii 5 parts of the same 



ocean floor, which may be roughly classed as 



sands, clays, and limestones. Sands formed near- 



to shore sometimes pass into grits and pebble- 

 beds. And the limestones, like the sands, some- 

 times alternate with clays in vertical succession, 

 where they pass horizontally into each other. In- 



ices may occur where limestones extend con- 

 tinuously from the shore to the open ocean, with- 

 out intervening deposits. 



The horizontal sequence of water-formed 

 rocks, observed at the present day, explains the 

 meaning of the vertical succession of the layers 

 of rock termed strata, which differ from each 

 other in mineral character. By their superposi- 

 tion they build up most of the visible land, as well 



