54 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



Cracks are common over all the harder parts of the reef. These cracks 

 run in all directions : some of them are parallel with the main axis of 

 the reef, some are at right angles to it, many of them are forked, and 

 some of them curve through an arc of ninety degrees within a distance of 

 forty metres. These cracks are not open ones as a rule, though some of 

 them are. Some of them gape broadly to a depth of four to six centi- 

 metres only. 



These cracks look as if they were worn, but I have never been able to 

 find any evidence of such wearing unless the form itself be accepted as 

 evidence. At many places the crack has the rock on one side of it five 

 or six centimetres higher than on the other, much as if the rock were 

 slightly faulted. It should be added that they are not faults. 



Some cracks that would otherwise be open at the surface are closed 

 by the ingrowing of Serpulae. These cracks are generally deep, but are 

 only seven centimetres or less in width. 



Fig. 27. Sections across typical cracks in a stone reef. 



Almost everywhere that the rocks are weathered or unprotected they 

 show false bedding, and less frequently they exhibit the true bedding. 

 In Plate 41 the true bedding can be plainly seen. 



The tide-pools over the surface of the reef contain fishes, crabs, and 

 the like. Of the corals only a few forms are found in these pools : most 

 abundant are Pontes and the small heads of Favia. 



There are no signs of a coral reef seaward of the Mamanguape sand- 

 stone reef. 



It is unfortunate that I was unable to see the southern end of the 

 Mamanguape reef and to learn something of its geologic relations there. 

 It is also to be regretted that the bearing of what could be seen at the 

 point where the reef approaches nearest the shore at Barra do Maman- 

 guape is not altogether clear, except as it can be explained by reference 

 to other places. 



As shown on the accompanying map, the great outer reef continues 

 southward past Mamanguape Point a kilometre or more, bearing south 

 G° west (magnetic) and parallel with the shore. Immediately west of 

 the reef, and running against it as if against a wall, is the south-flowing 



