Bar of Sandstone offPernambuco. 259 



calcareous matter ; this, on the outer subsided masses, which 

 can only be reached between the successively breaking waves 

 at low water, is so thick, that I could seldom expose the 

 sandstone with a heavy hammer. I procured, however, some 

 fragments where the layer was between three and four 

 inches in thickness; it consists chiefly of small Serpidce, 

 including some Balani, and a few very thin paper-like layers 

 of a Nidlipora. 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 exposed to the surf. In 

 the Pacific and Indian Oceans the outer and upper margin 

 of the coral reefs are protected, as will be described in a 

 forthcoming work, by a very similar coating; but there it is 

 almost exclusively formed of several species of Nulliporce. 

 Lieut. Nelson, in his excellent memoir on the Bermudas 

 (Geol. Trans., vol. v. part 1. p. 117), has described reefs, 

 formed, as he states, but I cannot avoid suspecting only coated, 

 by similar masses of Serpulce. I inquired from some old 

 pilots, whether there was any tradition of change in the form 

 and dimensions of this sandstone bar ; but they were unani- 

 mous in answering me in the negative. It is astonishing to 

 reflect, 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 centuries, 

 or more probably thousands of years. Seeing that the sur- 

 face on the inner side does gradually wear away, as shown by 

 the pebbles on the sandstone pedestals, this durability must 

 be entirely owing to the protection afforded by the thin coat- 

 ing of Serpulce and other organic beings: it is a fine example, 

 how apparently inefficient, yet how effectual, are the means 

 of preservation, like those of destruction, which nature em- 

 ploys. 



I believe similar bars of rock occur in front of some of the 

 other bays and rivers on the coast of Brazil : Baron Roussin 

 states that at Porto Seguro there is a "quay" similar to that 

 of Pernambuco. Spaces of several hundred miles in length 

 on the shores of the Gulf of Mexico, the United States, and 

 southern Brazil are formed by long narrow islands and spits 

 of sand, including very extensive shallow lagoons, some of 

 which are several leagues in width. The origin of these linear 

 islets is rather obscure: Prof. Rogers (Report to British 

 Association, vol. iii. p. 13.) gives some reasons for suspecting 



