THE SUCCESSION OF STRATA. 55 



result from waste of the land, succeed each other 

 vertically in reverse order. In the sea which 

 borders the coast of South America, the sand de- 

 rived from its cliffs may be carried out to a dis- 

 tance of from 20 to 150 miles from the shore. 

 The mud formed at the same time would be car- 

 ried much further. If then such a land were to 

 be enlarged by slow upheaval, so that the shore 

 extended over the area which had previously been 



the shallow sea, two sediments which would still 



be formed would be carried as great a distance as 

 they were carried previously, and the sand at its 

 furthest limit from the shore would gradually ex- 

 tend beyond the limit of the sand beneath it, and 

 would thus be super-imposed in part at least upon 

 clay. In like manner the mud sediment would 

 be carried further than the mud had gone pre- 

 viously, SO that it would similarly rest upon lime- 

 stone 



Therefore, under the influence of continual 



depression, the geological deposits come to be 



accumulated in the vertical order of sand, clay 

 and limestone, in the same place. While under 

 the influence of continued upheaval the vertical 



Succession comes to be limestone, clay, sandstone. 



There are constant oscillations of level of 

 land which are evidenced by successions of sand 

 and clay, or limestone and clay, which are local. 

 And occasionally a sediment is derived simul- 

 taneously from two different sources, as when 

 ancient cliffs furnish sand, and an ancient river 

 supplies mud which is deposited at the same time, 

 over part of the same area. 



The layers of water-formed rock which form 

 every land, succeed each other vertically in some 

 such order as sand, clay limestone, limestone, clay 



