128 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
mangrove swamps or tide marshes. To be specific, however, we shall 
| mention what are regarded as more or less satisfactory evidences of this 
depression. 
Open bays. — In all probability those geographers who have expressed 
the opinion that the Brazilian coast is one of recent depression have 
based their opinions upon the peculiar forms of the bays of Rio de | 
Janeiro and Bahia. These bays not only have many of the peculiarities 
of harbors formed by depressions, but may even be accepted as the very 
types of such harbors. 
Before the depression took place Bahia was a hilly, almost a moun- 
tainous region. Its streams were rapid, its valleys steep-sided and 
v-shaped; its hills were on the whole even-topped, but erosion had 
worked down among them until many of these were left as isolated 
peaks above the lowering valleys about them. Most of the streams 
flowed approximately where they now flow, but those now entering the 
bay united in the bottom of a broad valley and flowed out through what 
is now the mouth of the bay into the sea. All that is now known of | 
| the date of this erosion is that it was post-Eocene. | 
| When the great depression came, the Bahia valley sank and the waters | 
| of the ocean backed up into it and made the bay of Bahia very much as 
| we now see it. The scour of the tides has kept the entrance to the bay 
open, but the upper ends or indentations of the bay have been silted up 
with the aid of the encroaching mangroves. The tops of some of the 
hills were left sticking above the surface as islands. Some of these hills 
were cut off by the waves, and others were left as islands, and are now 
| known as Mare, Ilha dos Frades, Madre Deus, Ilha das Fontes, etc. 
| Itaparica and Ilha Santo Amaro became islands at the same time. | 
The Bay of Rio de Janeiro has had a similar history. Its basin was | 
carved out by subaerial erosion when the land stood at a higher eleva- | 
tion. It sank and water flowed over and filled the valley leaving it a 
| broad but not very deep bay. Some of the original islands of the bay 
| had only shallow waters between them and the mainland, and these shal- 
lows were eventually built up by the shore waste thrown upon them 
until the islands were joined to the land. The Pão d’Assucar case cited | 
on page 124 is an illustration of this process. 
The Bay of Santos, about 36.5 kilometres along the coast southwest of 
Rio de Janeiro, is an excellent illustration of an open island-filled bay 
smaller than that of Rio de Janeiro formed by a depression of the land. 
This bay is now almost entirely choked up by the silts washed into it 
from the surrounding mountains and thrown back into it from the coast. 
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