48 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
from which it is separated by only a few small breaks. At the south 
end also it continues far beyond the limits shown upon the accom- 
panying map, so that, long as it is represented, its total length is not 
really known, and neither are its geologic and geographic relations 
about its southern end. 
The Rio Mamanguape, а stream that drains a large area in Parahyba 
do Norte and in Rio Grande do Norte, flows down behind the reef and 
debouches both right and left through breaks or small bars, none of 
which lies in front of the river proper. The southern channel where it 
flows between the outer and inner reef is only about one hundred metres 
wide, but the channel is deep and the current is strong, 
The country landward is all flat and low back several kilometres to 
the base of the hills or table-lands that skirt all this northern coast. 
The bay itself is mostly quite shallow, and at low tide looks like a 
series of sand banks and shallow ponds. In the deep water of this bay 
the dredge brought up only sand and fragments of broken shells. 
The northern end of the reef that lies south of the Barra do Maman- 
guape is a single, flat, and nearly straight reef down to a point shown on 
the map, one and a half kilometres south of the bar, where it has the 
appearance of branching. Here a smaller and lower reef puts off from the 
main one and runs parallel with it and at a distance of about one hundred 
metres from it, until after several large breaks it joins the beach west 
of the river at Mamanguape Point. This inner reef is really a lower 
bed or beds of the main or outer reef. Where the two separate the 
inner one can be seen to dip gently beneath the great outer one. The 
junction of the two is fairly well shown in Plate 34. It is noticeable 
that while the lower reef is comparatively strong at this junction, it 
weakens southward as it separates from the larger one. The rock of 
the inner reef is of the same kind as that of the outer, save that it is not 
во hard. 
At high tide (flood tide, June 23, 1899) the big reef was only 0.45 to 
0.61 metres out of water at some of its highest points, while the surf 
broke over all of it save where isolated loose blocks have been piled on 
top of it. At low tide it stands from 2.1 to 2.4 metres out of water, 
Considered lengthwise, there is but little difference in the level of the 
top of the reef, — perhaps not as much as one metre in its entire length, 
breaks excepted. Here and there it has been undermined, and the sur- 
face rocks of harder stone have been Jet down in these gaps and are 
now covered with barnacles, seaweeds, and the like. These breaks are 
of various sizes, from those that one can walk across at low tide to those 
