208 THE VOYAGE OF H.M.S. CHALLENGER. 



applicable not only to the substance in question, but to all the concretionary phosphates found united 

 with the olivine rock of which the islets are composed. 1 



" Some brecciated specimens of St. Paul's Eocks are coated on both sides with black bands, 7 or 8 

 mm. thick, presenting the mineralogical characters of manganese. Sir Wy ville Thomson 2 describes 

 this breccia. Mr. Moseley points out that MacCormick had already drawn attention to this black 

 coating in the fissures of rock. Sir Wyville says that the coating, when triturated, gives a dirty-looking 

 greenish-grey powder, which effervesces in hydrochloric acid, setting chlorine free, and colouring 

 the acid in the same manner as protoxide of manganese. Moreover, Mr. Buchanan found in these 

 breccias with black incrustation, phosphate and carbonate of lime, carbonate of magnesia, and traces 

 of copper and iron, while the crust itself yielded water in the test-tube. I have been able to recognise 

 traces of manganese in unaltered specimens of the olivine rock. 



" With regard to the mode of formation of the rock, there are no other positive data than the 

 lithological. Lithological constitution alone cannot always decide the question of origin, the 

 uncertainty increases in proportion as new peridotic rocks are discovered, for fresh discoveries 

 frequently upset views previously entertained. It may be admitted in a general manner that 

 no objections can be raised, A priori, against the volcanic origin, pure and simple, of a peridotic 

 rock ; olivine can be crystallised artificially with the greatest facility by dry fusion. The igneous 

 origin of this mineral is also proved by its presence in the lavas of active volcanoes, and in older 

 rocks universally admitted to be pyrogenous. Not only can olivine, considered as a mineral, be 

 unquestionably igneous, but some peridotites, if we are to judge from the investigation of competent 

 observers, as Bonney, Hochstetter, &c, present positive characters of eruption. But while some 

 peridotites are eruptive, it is no less true that many masses of olivine rock present characters from 

 which an igneous origin canuot be demonstrated, 3 and it seems certain that very often true peridotites 

 do not occur in the form of injected veins. From the data collected iu the Report it is evident that 

 one may admit for the peridotic rocks two modes of origin, and that the question of origin is on the 

 whole to be decided rather by reference to the position of the rocks in relation to those among 

 which they lie, than by mineralogical composition. Unfortunately, however, this very important 

 element of the relation of the rock to those that encircle it is wanting in the case of St. Paul's 

 Rocks. The rocks stand alone in mid-ocean, and of their connection with other rock masses we 

 can state nothing definite. 



" The reason that pleads in favour of the eruptive theory is the law of analogy. We know 

 indeed that the small oceanic islands are either of volcanic or coral formation. May not the peridotite 

 of St. Paul's Rocks be assimilated to the group of crystalline rocks represented by the syenite, 

 diabase, and melaphyres forming the base of several volcanic islands of the Atlantic? An obvious 

 argument in favour of an eruptive origin is afforded, by the fact that the bottom of the Atlantic has 

 been for long ages, in many points, the theatre of volcanic manifestations ; and in particular, the 

 region in which St. Paul's Rocks are situated has in comparatively recent times shown signs of 

 eruptive phenomena. The isolation of these rocks might be adduced as a further proof of their 

 eruptive origin. The soundings taken between St. Paul's Rocks and the nearest continent and other 



1 See analysis of a decomposed specimen impregnated by phosphate of lime, in Narr. Chall. Exp., vol. ii., App. B., 

 p. 18, 1882. 



a The Atlantic, voL ii. p. 106. 



3 See the resume 1 of the results arrived at by the observers who have described peridotites found as regular inter- 

 calations in various formations, in Narr. Chall. Exp., vol. ii., App. B, p. 24, 1882. 



