52 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



there it is as flat and smooth as any sandstone slab for hundreds of 

 metres ; again it is so rough and uneven that it is almost impossible to 

 walk over it. Over much of the surface there are shallow depressions 

 that look like gigantic footprints made by mud-clogged feet in thick 

 mud. At such places the surface of the reef has a profile like this : 



Fig. 22. Section across the Mamanguape stone reef. 



In plan these pits have these and similar forms (Fig. 23) : 



These pits are from three to seven 

 centimetres deep and from a few centi- 

 metres to two metres long. For the 

 most part they are parallel, but some- 

 times they stand at various angles to 

 each other. 



The Mamanguape reef has some fine 

 examples of etched surfaces. One of 

 these is shown in Plate 41, a photo- 

 graph taken near the inner edge of the 

 reef on its southern half. This etching leaves ragged sharp points that 

 vary in height from a few centimetres to a metre, and a great number of 

 fantastic forms. 



Fig. 23. Forms of the pits on 

 surface of the Mamanguape reef. 



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Fig. 24. Characteristic forms produced by the etching of the Mamanguape 



stone reef. 



Some of the pillars on the Mamanguape reef are as much as two 

 metres high ; the tall ones are, as a rule, on the landward side of tiie 

 reef, — never close to the surf-beaten sea side. These spike-like projec- 



