268 APPENDIX. 



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 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. This is a fine example of what apparently 

 inefficient means may be effectual. 



I believe that 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 of Southern Brazil are formed by long narrow islands and 

 spits of sand, including extensive shallow lagoons, some of 

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

 islets is rather obscure : Professor Rogers (Report to British 

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

 they have been formed by the upheaval of sand-banks, deposited 

 w r here currents formerly met. The bar of sandstone at Per- 

 nambuco has probably been formed in an analogous manner. 

 The town stands partly on a low narrow islet and partly on a 

 long spit of sand, in front of a low shore, bounded in the dis- 

 tance by a semicircle of hills. By digging at low r water near 

 the town, the sand is found consolidated into sandstone, similar 

 to that of the bar, but containing many more shells. If, then, 

 the nucleus of a spit of sand, extending in front of the bay, 

 had formerly become consolidated, a small change, probably of 

 level, but perhaps merely in the currents, might have given 

 rise, by washing away the loose sand, to a structure like that 

 of the bar in front of Pernambuco and along the coast south- 

 ward of it ; but without the protection afforded by the succes- 

 sive growth of the above-named organic beings, its duration 

 would have been short. 



