258 Mr. Darwin on a remarkable 



The accompanying woodcut represents, at low water spring 

 tides, a transverse section of the northern part of the bar, 

 where a section of about seven feet in height is exhibited on the 

 inner side. It consists of a hard pale-coloured sandstone, break- 

 ing with a very smooth fracture, and formed of siliceous grains, 

 cemented by calcareous matter. Well-rounded quartz pebbles, 

 from the size of a bean, rarely to that of an apple, are im- 

 bedded in it, together with a very few fragments of shells. 

 Traces of stratification are obscure, but there was an included 

 layer in one spot ofstalactitic limestone, an eighth of an inch in 

 thickness. In another place some false strata, dipping land- 

 wards at an angle of 45°, were capped by a horizontal mass. 

 On each side of the ridge quadrangular fragments have sub- 

 sided, as shown in the woodcut; and the whole mass is in some 

 places fissured, apparently from the washing out of some soft 

 underlying bed. One day, at low water, I walked a full mile 

 along this singular, smooth, and narrow causeway, with water 

 on both sides of me, and could see that for nearly a mile 

 further its form remained unaltered. In Baron Roussin's 

 beautiful chart of Pernambuco (Le Pilote duBresil) it is re- 

 presented as stretching on, in an absolutely straight line, for 

 several leagues; how far its composition remains the same, I 

 know not; but from the accounts I receive^ fi'om intelligent 

 native pilots, it seems to be replaced on some parts of the 

 coast by true coral reefs. 



The upper surface, though on a large scale it must be 

 called smooth, yet presents, from unequal disintegration, nu- 

 merous small irregularities. The larger imbedded pebbles 

 stand out supported on short pedestals of sandstone. There 

 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; they end abruptly, but in a rounded form. One 

 of the furrows occasionally branches into two arms, but ge- 

 nerally they are nearly parallel to each other, and placed in 

 lines transverse to the sandstone ridge. I know not how to 

 account for their origin, without .they be formed by the surf, 

 as it daily breaks over the bar, washing to and fro pebbles in 

 depressions, originally only slight. Opposed to this notion is 

 the fact, that some of them were lined with numerous small 

 living ActinecE. I have copied this passage, as I at the time 

 wrote it, because furrows of a somewhat similar nature on the 

 surface of rocks have lately received much attention, and are 

 supposed invariably to indicate the former action of a water- 

 fall, over the edge of a moving glacier. 



The exterior part of the bar is coated with a thin layer of 



