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 Eio Mamanguape, a stream that drains a large area in Parahyba 

 do Xorte 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 ofi" from the 

 main one and runs parallel with it and at a distance of about one hundred 

 metres from it, until after sevei'al 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 

 so 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 let 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 



