REPLACING OVERLAP 



743 



region of Pennsylvania, but these organisms do not necessarily in- 

 dicate marine conditions. 



c. REPLACING OVERLAP. This type of overlap occurs 

 when a series of clastic sediments of terrestrial origin replaces a 

 typical oceanic or thalassic series, or where continental elastics re- 



e-j;»i° 



Rockes^f 



Fig. 157. Diagram showing 'the westward replacing overlap of the Utica 

 shale on the Trenton limestone, both being marine formations, 

 but one (the Utica) terrigenous and the other thalassigenous. 



place those formed in the sea (Fig. 155). The latter is associated 

 only with a retreating seashore, and the overlapping of the nonma- 

 rine beds does not take place on the old land surface, but upon previ- 

 ously deposited and all but contemporaneous marine beds. Along 

 the border line the two will blend, and it will appear as if the non- 



AppaluMaa Reglm 



Marine Series 



Fig. 158. 



Continental Series 



Diagram showing replacing overlap of terrigenous marine, fol- 

 lowed by continental sediments on the east over marine lime- 

 stones, etc., on the west. 



marine series overlay the marine series, though in reality it is a case 

 of replacement of the one by the other. A typical example of such 

 a progressive replacement is seen in the Catskill sandstone, which 

 as a nonmarine formation spread westward from its centers of ori- 

 gin, of which it appears there were at least two. The correspond- 



