BAR OF SANDSTONE. 
279 
calcareous matter ; this, on the outer subsided masses, 
which can be reached only at low water, between the 
successively breaking waves, is so thick that I could 
seldom expose the sandstone by the aid of a heavy hammer. 
I procured, however, some fragments, which were between 
three and four inches in thickness, and consisted chiefly of 
small Serpulce, including some Balani , with a few thin 
paper-like layers of a Nullipora. The surface alone is alive, 
and all within consists of the above organic bodies, filled up 
with dirty white calcareous matter. The layer, though not 
hard, is tough, and from its rounded surface resists the 
breakers. Along the whole external margin of the bar, I 
only saw one very small point of sandstone which was ex- 
posed to the surf. In the Pacific and Indian Oceans the 
outer and upper margin of the coral-reefs are, as we have 
seen, protected by a similar coating ; but formed almost 
exclusively of several species of Nullipora ?. Lieut. Nelson, 
in his excellent memoir on the Bermudas (Geol. Trans, 
vol. v. part 1, p. 117), says that the reefs there are formed 
of similar masses of Serpulce ; but I suspect that they are 
only thus coated. 
I enquired from some old pilots at Pernambuco whether 
there was any tradition of the bar having undergone any 
change during the lapse of time ; but they were unanimous 
in answering me in the negative. It is astonishing to re- 
flect, that although waves of turbid water, charged with 
sediment, are driven night and day by the ceaseless trade 
wind against the abrupt edges of this natural breakwater, 
yet that it has lasted in its present perfect state for cen- 
turies, or perhaps for thousands of years. Seeing that the 
surface on the inner side does gradually wear away, as 
shown by the pebbles on the little sandstone pedestals, this 
durability must be entirely due to the protection afforded 
by the thin coating of Serpulce and other organic bodies. 
