46 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
sand which everywhere has the appearance of having been blown up 
from the beach. In places this sand ridge is scarcely high enough to 
keep the spring tides from flowing over it into the lake, but at one place 
south of the village it has a height of nine metres on the seaward side. 
From the village northward to the base of the hill on which the old fort 
stands this ridge is only from sixty to one hundred and twenty metres 
wide from fresh water to highest tide-level on the sea side. South of the 
village it widens out somewhat, and at one place is two hundred metres 
wide. The drainage of the lake is thus compelled to seek an outlet 
through the Rio Mamanguape, ten kilometres south of the low narrow 
neck north of Traição. 
Plate 30 shows this neck of sand with the bay to the right and the 
marsh of Lagóa de Sinimbú on the left. 
A line of levels run across this neck shows that the lake water on July 
24, 1899, was fifty-nine centimetres lower than the level pointed out as 
that of the highest spring tides on the sea beach. The mean tide- 
level, however, is a metre or more lower than the lake surface, 
The Lagóa de Sinimbú and the flat lands about it and about the 
mouth of the Mamanguape lie as one broad flat region behind the reefs 
of Traição and Mamanguape alike. For a fuller discussion of these 
geographic features and their history and bearing upon the reefs the 
reader is referred to Chapter V., pp. 111 to 170. 
The Traição end of the reef ends rather abruptly, standing boldly out 
to sea. At low tide one can walk dry-shod from Traição Point out on the 
stone reef, so that it may be said to join the land at that place. South of 
Traição Point the land draws away from the reef, forming thus the north- 
ern part of Mamanguape Bay, while north of the point the land swings 
away still more sharply to form the Bahia de Traigäo. A good view of 
the bay is had from the site of the old abandoned fort north of the town ; 
from this point the photographs for Plates 29 a and 29 b were taken. 
The Traição reef has a total length of only a little more than two 
kilometres ; this does not include the fragments or outliers that connect 
this with the Mamanguape reef. Those fragments alone down to the 
bar have a total length of 2.4 kilometres. 
A glance at the map shows that Traição reef has a gentle outward 
curve at Traição Point. It is rather broken, and its outer edge is 
ragged. In several places the waves are undermining it from the out- 
side. On the whole it is a flat and rather smooth reef. Along the 
inner margin are the usual points left by surface etching, but they are 
not so high or so prominent a feature of this as of the Mamanguape 
