REPORT ON THE PETROLOGY OF THE ROCKS OF ST. PAUL. 



19 



Analysis V. belongs to a breccia of very peculiar character, in which are distinguished 

 fragments of shells and debris of the bones of vertebrates, enclosed in a compact greyish 

 earthy mass. 



V. (B). 



4-90 



Loss by heat at 230° Fahr., 



Oxide of iron, 



Phosphate of lime, 



Protoxide of iron, 



Oxide of manganese, 



Sulphate of lime, 



Carbonate of lime, 



Magnesia, 



Silica, 



Insoluble residue, 



1-45 



38-40 

 traces 

 0-50 

 2-90 

 33-38 

 9-37 

 7-70 

 1-40 



100-00 



This breccia is bordered on both sides by black bands, 7 or 8 millimeters thick, 

 presenting all the mineralogical characters of manganese. Sir Wyville Thomson 1 

 describes a breccia similar to the one analysed above, " Each face of the crevice," 

 he says, " is covered by a hard black coating about a quarter of an inch thick." 

 Moseley also points out that MacCormick 2 had already drawn attention to this black 

 coating of the rock-fissures. Sir Wyville Thomson says that the coating when tritur- 

 ated gives a dirty-looking greenish-grey powder, which effervesces in dilute hydro- 

 chloric acid, leaving a brown insoluble residue, and when treated with concentrated 

 hydrochloric acid, it set the chlorine free, and coloured the acid in the same manner 

 as protoxide of manganese. Moreover, Mr. Buchanan 3 found in these breccias with 

 black incrustations, phosphate and carbonate of lime, carbonate of magnesia, traces of 

 copper and of iron, while the crust itself yielded water in the test-tube. 



Although there is nothing more common than to find coatings of manganese deposited 

 by infiltration in the crevices of rocks of older formations, as well as in modern 

 superficial deposits, yet before the Challenger expedition no one suspected the important 

 part played by this mineral in deep-sea deposits. Without stopping here to discuss 

 the formation of the nodules of manganese at the bottom of the sea, we may 

 mention as a characteristic of this mineral that it forms concretionary deposits in 

 the fissures of silacid rocks, of whose decomposition it is a result. I have been 

 able to recognise traces of manganese in unaltered specimens of the olivine rocks of 

 St. Paul. 



Analysis VI. was made of a fragment of scoriaceous aspect, having somewhat 



1 Sir Wyville Thomson, Voyage of the Challenger, vol. ii. p. 106. 



2 Moseley, Notes by a Naturalist on the Challenger ; MacCormick, Voyage (Ross's) to the Antarctic and Southern 

 Regions, vol. i. pp. 14-18. 



3 Buchanan, Proc. Roy. Soc, vol. xxiv., No. 170, 1876, p. 613. 



