BAR OF SANDSTONE. 267 



are, also, many sinuous cavities, two or three inches in width 

 and depth, and from six inches to two feet in length. The 

 upper edges of these furrows sometimes slightly overhang their 

 sides ; and they end abruptly with a rounded outline. A 

 furrow occasionally branches into two arms, but generally they 

 run nearly parallel to each other, in a line transverse to the 

 sands:one ridge. I know not how to account for their origin 

 except through the washing to and fro of pebbles in originally 

 slight depressions, by the waves which break daily over the 

 bar. Opposed to this notion is the fact that some of these 

 fuiTows were lined with numerous small living Actinece. The 

 exterior surface of the bar is coated with a thin layer of cal- 

 careous matter ; this, on the outer subsided masses, which can 

 be reached only at low water, between the successively break- 

 ing waves, is so thick that I could seldom expose the sand- 

 stone 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 exposed 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 Nulliporce. Lieut. Nelson, in his excel- 

 lent memoir on the Bermudas (Geol. Trans, vol. v. part 1, 

 p. 117), says that the reefs there are formed of similar masses 

 of Serpuloa ; 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 reflect, 

 that although waves of turbid water, charged with sediment, 



