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bulletin: museum of comparative zoology. 



places will eventually give way and form gaps or barretas in the 



reef. 



That the surf has upon occasion been able to break off large blocks 



from the reef is shown by those now found lying loose on the surface at 



several places. One of these blocks is 

 estimated (at one hundred and sixty 

 pounds to the cubic foot — a low esti- 

 mate) to weigh not less than nineteen 

 tons. Another weighs five tons, and 

 still another weighs twenty-two tons. 

 This last one has been swept over 

 and across the reef and now lies close 

 to its inner edge. The live-ton block 

 has its upper surface striated and pol- 

 ished very much as if it had been 

 glaciated. This has been produced 

 by its having been pushed gradually 

 by repeated blows across the reef; 

 when near the inner margin it was 

 turned completely over and left 

 where it now lies. 



At another place north of the fork 

 in the reef there are thirteen blocks 

 or slabs, some of them weighing sev- 

 eral tons, lying on top near the inner 

 margin and within a distance of a 

 hundred metres. Some of these pieces 

 show by the position of the fossil 

 shells they contain that they have 

 been inverted by the waves. ^ 



The rock of the reef is a slightly 

 yellowish, rather coarse, but remark- 

 ably fresh-looking sandstone. In 

 places it contains beds of pebbles, 

 but these beds are neither thick nor 

 wide-spread. Near the middle of the 



i-eef there is exposed on the surface a bed of quartz pebbles many of 



1 The majority of separated bivalve shells lie on a beach in such a position as 

 to offer tlie least resistance to the water passing over them; that is, with the con- 

 vex side upward. 



Fig. 21. Characteristic breaks of the 

 outer edge of the Mamanguape 

 stone reef. 



