COMPOUND OVERLAP 



m 



divisions a to d, are both complete in section 5, which marks the 

 farthest Hmit of retreat of the sea. Bed x, the intercalated shore 

 formation, fades away in this section. In section 4 all the members 

 of both formations are represented, but divisions 4 and a are not 

 complete, while the intercalated bed x is prominent. In section 3, 

 divisions 4 and a are wanting, and in section 2. divi'^ions j and h are 

 likewise wanting. In section i. finally, all except the lowest divi- 

 sion (/) of formation A, and the highest (ri) of formation B. are 

 missing. It would be obviously wrong to correlate division / of sec- 

 tion I with the whole of formation A as exposed in section 5, or 

 division d of section i with the whole of formation B of section 5. 

 Yet such correlations have not infrequently been made in the past. 

 In each of the above sections the intercalated sandstone or con- 

 glomerate may be intimately related to both overlying and underly- 

 ing formations, and contain their fossils. Such a relation would 



Fig. 150. Diagram of a compound regressive-transgressive series, showing the 

 seaward decrease of the hiatus between the upper and lower 

 part of the series. (See Fig. 149.) 



show that the actual hiatus is represented by the middle part of the 

 intercalated bed, which middle part may not infrequently be repre- 

 sented by continental deposits. Yet so intimate are the physical re- 

 lationships between the bottom, middle and top of the intercalated 

 formation that a separation into more than one formation is im- 

 possible. Thus in section i the intercalated formation x may have 

 all the characters of a formational unit or a single stratum, and may 

 grade upward into division d and downward into division /, so that 

 to all appearance continuous deposition obtained in this locality. If 

 the respective ages of divisions d and / are recognized, formation x 

 will be likely to be considered the depositional equivalent of divi- 

 sions 2 \o 4 and a to r of section 5. Such is indeed the fact, except 

 that the central portion of .r, the terrestrial portion, whether repre- 

 sented by continental deposits or an unrecognizable erosion plane, 

 represents all but the upper fraction of division c, and the lower of 

 division 2. The hiatus or stratigraphic break thus represented in 



