446 STUDIES FOR STUDENTS 



deposits and comprises the overlying formation; frequently absent 

 where the old land surface, ordinarily a land erosion surface, under- 

 lies the marine deposits. The littoral zone is small in local area 

 compared with the occasional delta deposits which border it on the 

 one hand, and the universal marine deposits which border it on the 

 other. As seen in vertical section, therefore, the littoral deposits 

 should have but limited lateral development on any one horizon. 

 If the movement has been the subsidence of an old land, they should 

 not be again encountered in the superior strata, but in the case of 

 the contact of a delta with an epicontinental sea the littoral zone, 

 as shown by the diagram, Fig. lo, might move backwards and for- 

 wards as a zigzag line through the vertical section, representing the 

 advance and retreat of the sea as the subsidence on the one hand, 

 or delta-building on the other, dominated at the moment, the littoral 

 deposits dividing the marine from those made upon the surface of 

 the land. 



RELATIONS OF CONTINENTAL AND MARINE SEDIMENTATION 

 THROUGH GEOLOGICAL TIME 



In the preceding discussion the relative importance in area and 

 volume of modern land, seashore, and shallow-water marine deposits 

 has been discussed. It has been seen that the littoral zone occupies 

 the least area and stands 'the smallest chance of prcserval, while 

 land deposits of various sorts hold second place; most important 

 among these, on account of their favorable chances for preserval, 

 being river sediments, made in interior basins, or as deltas encroach- 

 ing upon the sea. The marine detrital deposits of the continental 

 margins are, however, the most widespread and important of the 

 three classes. 



In discussing these relations they were seen to be dependent upon 

 many broad features, such as the degree of continental uplift and 

 areal extension, recency and magnitude of mountain movements, 

 climatic zones, and other such factors. These have varied widely, 

 however, through past time, and while the geographic details of for- 

 mer ages are largely wanting, still the general character of the ter- 

 restrial surface is in a manner known ; as, for example, it is known 

 that the Triassic over every continent was in general characterized 



