THE VOYAGE OF THE BEAGLE 525 



obscurely stratified hard sandstone. At high water the waves 

 break over it; at low water its summit is left dry, and it 

 might then be mistaken for a breakwater erected by Cyclo- 

 pean workmen. On this coast the currents of the sea tend 

 to throw up in front of the land, long spits and bars of 

 loose sand, and on one of these, part of the town of Per- 

 nambuco stands. In former times a long spit of this nature 

 seems to have become consolidated by the percolation of 

 calcareous matter, and afterwards to have been gradually 

 upheaved ; the outer and loose parts during this process hav- 

 ing been worn away by the action of the sea, and the solid 

 nucleus left as we now see it. Although night and day the 

 waves of the open Atlantic, turbid with sediment, are 

 driven against the steep outside edges of this wall of stone, 

 yet the oldest pilots know of no tradition of any change in its 

 appearance. This durability is much the most curious fact 

 in its history : it is due to a tough layer, a few inches thick, 

 of calcareous matter, wholly formed by the successive 

 growth and death of the small shells of Serpulae, together 

 with some few barnacles and nulliporae. These nulliporae, 

 which are hard, very simply-organized sea-plants, play an 

 analogous and important part in protecting the upper sur- 

 faces of coral-reefs, behind and within the breakers, where 

 the true corals, during the outward growth of the mass, 

 become killed by exposure to the sun and air. These in- 

 significant organic beings, especially the Serpulae, have done 

 good service to the people of Pernambuco ; for without their 

 protective aid the bar of sandstone would inevitably have 

 been long ago worn away and without the bar, there would 

 have been no harbour. 



On the iQth of August we finally left the shores of Brazil. 

 I thank God, I shall never again visit a slave-country. To 

 this day, if I hear a distant scream, it recalls with painful 

 vividness my feelings, when passing a house near Pernam- 

 buco, I heard the most pitiable moans, and could not but 

 suspect that some poor slave was being tortured, yet knew 

 that I was as powerless as a child even to remonstrate. I sus- 

 pected that these moans were from a. tortured slave, for I 

 was told that this was the case in another instance. Near 

 Rio de Janeiro I lived opposite to an old lady, who kept 



