7Z^ 



PRINCIPLES OF STRATIGRAPHY 



As the shore zone advances landward, finer deposits Will be laid 

 down over the coarser basal bed and thus the transgressive portion 

 of such a sandstone or conglomerate will pass diagonally across the 

 planes of synchroneity. The deposit resulting by the time the 

 shore zone has returned to its former position will therefore be 

 a composite formation including within it a hiatus, which may rep- 

 resent a considerable time interval, but is not recognizable by any 

 structural character. The encroaching sea will work over the sand 

 dune deposits and thus water-laid sandstone beds, composed of 

 rounded translucent quartz grains and well stratified, will result. 

 In the upper part of these worked-over sandstones marine shells 

 and other remains may be included, the age of which changes 

 shoreward, since the transgressive portion of this sandstone repre- 

 sents successively the ends of higher and higher formations, after 

 the manner of a true basal sandstone of a transgressive series. 



Fig. 149. Diagram showing the relationship of the strata in five successive 

 sections, in a compound regressive-transgressive series. The 

 intercalated sandstone x y encloses the hiatus. (See Fig. 150.) 



The recognition of an intercalated shore formation of great 

 areal extent between ofi^-shore sediments as the product of combined 

 regressive and transgressive movement must be based on a com- 

 parison of sections taken at intervals over the entire area covered 

 by the formation in question. Such sections will show the inter- 

 calated shore formation (sandstone or conglomerate) to be in 

 more or less intimate association wath the lower beds of the under- 

 lying formation and the higher beds of the overlying formation at 

 their shoreward ends, but, away from the shore, higher members of 

 the lower formation and lower members of the higher formation 

 will progressively appear until near the line of farthest retreat 

 both lower and upper formation wall be complete, while beyond that 

 the intercalated bed will gradually lose its . shore character and 

 merge with the enclosing formations. This character of the sec- 

 tions is shown in the preceding diagram (Fig. 149). Formation 

 A, consisting of divisions / to 4, and formation B, consisting of 



