ST. PAUL'S ROCKS. 75 



The group consists of five peaks of rock, disposed in four 

 principal masses which are separated by three narrow channels, 

 through which the surf perpetually roars and boils ; over one of 

 these channels it is possible to cross at low water, the tide 

 rising and falling here about five feet. The rocks are disposed 

 in a sort of horse-shoe round the bay; they are composed of 

 hard black rock, and another yellowish rock with black laminse 

 in it, " full of variously coloured pseudo fragments," according 

 to Darwin a variety of the former black rock. 



There are in places bands of a green stone resembling Ser- 

 pentine. The whole is intersected by various veins, mostly 

 nearly vertical and running in all directions, consisting of 

 various rocks, viz. : brown ferruginous lamina?, a coarse con- 

 glomerate of beach pebbles, and a finer conglomerate which 

 contains fragments of sea shells and nullipores, and which are 

 considered by Darwin as evidently of later origin than the 

 main mass of the rocks. These seams of conglomerates have 

 the appearance of having been formed of beach fragments 

 washed into fissures in the rock and consolidated there. Each 

 face of the containing fissure is covered by a peculiar dense and 

 hard black layer of about a quarter of an inch in thickness. 

 This black layer is mentioned by Mr. M'Cormick in " Eoss's 

 Voyage"; Mr. Buchanan found it to be composed of "phosphate of 

 lime, peroxide of manganese, a little carbonate of lime and 

 magnesia, with traces of copper and iron."* He considers that 

 the rocks as a whole may be classed as Serpentine. 



Mr. Darwin has dwelt on the importance of the fact that 

 the rocks are not volcanic, like nearly all other oceanic islands. 

 The depth to the eastward of St. Paul's Eocks is irregular, and a 

 depth of only 1,500 fathoms was obtained shortly before we 

 approached them, succeeded by deeper water. There is no con- 

 necting ridge between the rocks and Fernando do Norhona. No 

 doubt the rocks are the remnants of a much larger tract of land 

 now submerged, probably once continuous with these irregular 

 masses in their neighbourhood, and which may have had a 

 vegetation of its own. 



* J. Y. Buchanan, "Proc. B. Soc," No. 170, 1876, p. 613. 



