50 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 five-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 
Fig. 21. Characteristic breaks of the places it contains beds of pebbles, 
outer edge of the Mamanguape 
stone reef. 
but these beds are neither thick nor 
wide-spread. Near the middle of the 
reef 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 а position ав 
to offer the least resistance to the water passing over them; that is, with the con- 
vex side upward. 
