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



"Near this spot the officers of H.B.M.S. Challenger took magnetic observations, August 

 29th, 1873, dip 22-32. Caught plenty of fish." 



A few successful dredgings were obtained by laying out the dredge in a boat astern 

 of the ship while secured to the rocks, and heaving it back. 



The soundings close to St. Paul's Rocks showed a hard or rocky bottom, or a Globi- 

 gerina ooze containing numerous fragments of the rocks and olivine, enstatite, serpentine, 

 magnetic grains, and actinolite. 



A detailed Report on the Petrology of St. Paul's Rocks has been published in 

 Volume II. of the Narrative of the Cruise, 1 to which the Reader is referred for details. 

 The following note, giving the chief results of the investigation, has been furnished by 

 Professor Renard, F.G.S. : — 



" The position of St. Paul's Rocks, far removed from any continent, together with their aspect 

 and lithological characters, caused them to be considered as the last trace of some vast district lost 

 by submergence. Darwin, struck by the peculiar character of the mineral mass, deuied its volcanic 

 origin. He says : 2 — ' It is not of volcanic origin, and thi3 circumstance, which is the most 

 remarkable in its history (as will hereafter be referred to), properly ought to exclude it from the 

 present volume.' Speaking of the lithological character of these islets, he described tbem as com- 

 posed of rocks unlike any which he ever met with, and would not characterise them by any name. 

 He considered the northern rock of the group to be formed of a sort of " harsh stone," which breaks 

 up into fragments so regular as to be mistaken for blocks of altered orthoclase, and, moreover, saw 

 what he considered to be veins of serpentine running through the whole mass. The observers of the 

 Challenger Expedition, following Darwin, classed the rocks composing the group as serpentine. In 

 doing so they have placed them very nearly in the class they should occupy in the lithological series. 

 Mr. Buchanan ascertained during the voyage that the rock contained magnesia, alumina, and 

 peroxide of irou, and that many specimens gave off water on heating in a closed tube. The naturalists 

 who have visited the island have drawn attention to the fact that the rocks to the south are covered 

 over with a substance that gives them at a distance a dazzling white appearance. This is due in part 

 to the excrement of an immense multitude of seabirds that gather on the rocks, and in part to a 

 coating of a white, hard, brilliant material which will be described hereafter. 



" The olivine rock of St. Paul's Rocks presents in general an unusually fresh appearance ; showing 

 signs of decomposition only along the crevices. This peridotite is perfectly homogeneous to the naked 

 eye and very compact. Its colour is blackish-grey, bordering to green and black ; splinters of the rock 

 are translucent on the edges and of a greenish tint. The lustre varies from subvitreous to resinous; the 

 splinters redden before the blowpipe, and are infusible ; the streak is grey or greenish ; in hardness it is 

 inferior to felspar. An analysis by Dr. Sipocz has given SiO,, 43"84 ; Al 2 O g , 114; Cr 2 3 , 0-42 ; FeO, 8"76 ; 

 MnO, 0-12; NiO, 051 ; CaO, 171 ; MgO, 44-33 ; H 2 O,l-06 = 101-89. On calculating this analysis we find 

 that the rock contains 75 per cent, of olivine and 25 per cent, of enstatite. Thin sections from 

 slightly decomposed specimens show that the rock is composed of olivine, enstatite, and chromic iron. 



1 Report on the Petrology of St. Paul's Kocks, by the Rev. A, Renard, Narr. Chall. Exp., vol. ii., A pp. B. 



2 Darwin, Volcanic Islands, p. 32, 1851. 



