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 movements. 
Inasmuch ав the elevation recorded appears to have been a compara- 
tively small one, two orders are possible: — 
lst. 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 : — 
Fic. 90. Section across a mangrove swamp near São Thomé. 
Сава I. Section across the north of São Thomé, mangrove swamp 
and shell beds. See pp. 149-152. 
Hypothesis I. 
lst. Greater elevation. 
24. 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 first 
deposited, which of itself requires a 
still earlier depression. 
Hypothesis IT. 
18, Greater depression. 
9d. 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. 
Слзв П. Section across the São Thomé valley. See рр. 149-152. 
Hypothesis 1. 
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 IT. 
The greater depression would allow 
the shell bed to be formed after the 
depression, and slightly elevated later. 
