CHAP, u.] MJDElIiA TO THE COAST OF BRAZIL. 99 



in closed tiil)es, and consist chiefly of hydrated oxide of magne- 

 sia, with alumina and peroxide of iron in subordinate quantity. 

 Of the more recent veins mentioned by Darwin, some are bor- 

 dered on both sides by black bands of a hard infusible substance. 

 The pow^der has a dirty grayish-green color, and effervesces with 

 dilute hydrochloric acid, leaving a browni insoluble residue. In 

 strong hydrochloric acid it dissolves with evolution of chlorine, 

 and the color phenomena of dissolving peroxide of manganese. 

 It was found to consist of phosphate of lime, peroxide of man- 

 ganese, a little carbonate of lime and magnesia, and traces of 

 copper and iron ; like the other rocks, it gives off alkaline water 

 in a closed tube. Mr. Buchanan is inclined to regard all the 

 rocks as referable to the serpentine group. So peculiar, how- 

 ever, is the appearance which it presents, and so completely and 

 uniformly does the phosphatic crust pass into the substance of 

 the stone, that I felt it difficult to dismiss the idea that the 

 whole of the crust of rock now above water might be nothing 

 more than the result of the accumulation, through untold ages, 

 of the insoluble matter of the ejecta of sea-fowl, altered by ex- 

 posure to the air and sun, and to the action of salt and fresh 

 water, but comparable with the " stalactitic or botroydal masses 

 of impure phosphate of lime " observed by Mr. Darwin at As- 

 cension. "The basal part of these had an earthy texture, but 

 the extremities were smooth and glossy, and sufficiently hard 

 to scratch common glass. These stalactites appeared to have 

 shrunk, perhaps from the removal of some soluble matter in 

 the act of consolidation, and hence they had an irregular form." 

 The composition of the minerals at St. Paul's Rocks did not 

 seem, however, to be consistent wnth this mode of production. 



On the morning of the 29th we landed a party of explorers 

 and fishermen, and then cast off the hawser and went round the 

 rocks taking soundings and swinging for the errors of the com- 

 passes ; and in the evening, after picking up our sti*agglers, we 

 proceeded under all plain sail toward Fernando Noronha. 



A^•^^ 



