162 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY, 



Time Relations of the Elevations and Depressions. 



Having presented evidence of both an elevation and a depression, it 

 remains to determine the order of these movement.s. 



Inasmuch as the elevation recorded appears to have been a compara- 

 tively small one, two orders are possible : — 



1st. An elevation greater than the one here described, followed by a 

 depression that returned the elevated coast to within a few metres of 

 its original position. 



2d. A larger depression followed by a smaller elevation. 



These two assumed orders of events can be tested by a process of 

 elimination by trying to explain thereby the conditions known to exist. 



A section across one of the mangrove swamps shown on p. 150 is given 

 herewith : — 



Fig. 90. Section across a mangrove swamp near Sao Thome'. 



Case I. Section across the north of Sao Thome, mangrove swamp 

 and shell beds. See pp. 149-152. 



Hypothesis I. 



1st. Greater elevation. 



2d. Smaller depression. 



The elevation would allow the ero- 

 sion of the shell beds; the depression 

 would admit mangroves into the sub- 

 sequently drowned valleys. This re- 

 quires that the shell beds be fii-st 

 deposited, which of itself requires a 

 still earlier depression. 



Hypothesis II. 



1st, Greater depression. 



2d. Smaller elevation. 



The shells are deposited during the 

 depression ; the small elevation brings 

 them up where they are now being 

 eroded. In the swamps, the shallow 

 bay bottom is brought up within reach 

 of the plants. 



Case II. Section across the Sao Thome valley. See pp, 149-152, 



Hypothesis I. 



The earlier elevation would have to 

 assume the shell bed already formed, 

 in which case the elevation would 

 leave it to be eroded and depressed 

 again. 



Hypothesis II. 



The greater depression would allow 

 the shell bed to be formed after the 

 depression, and sUghtly elevated later. 



