Chap, iii.] GEOLOGICAL STRUCTURE. 65 



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

 hard black rock, and another yellowish rock with black laminae 

 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 laminae, 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 " Ross's 

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

 phate 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 Rocks is irregular, and 

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

 approached them, succeeded by deeper water. There is no 

 connecting ridge between the rocks and Fernando Noronha. 

 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. 



With regard to the present vegetation, as stated by Darwin 

 and Ross, there are no aerial plants on the rocks, not even a 

 lichen ; I found, however, a microscopic alga {Protococcus affinis), 

 growing on the guano in sheltered places and colouring it of a 

 dull green. In the stagnant pools on the rocks grow two low 

 green algse, Prasiola minuta and Oscillaria sordida, and a few 

 diatoms. 



The rocks are poorly supplied with the larger species of 

 seaweeds, apparently because these are unable to endure the 

 constant heavy surf. The high tide-mark is formed by a band 

 of a pinkish white nullipore {Lithothamnion polyinorphum) ; its 

 calcareous masses form an incrustation on the rocks, in places 

 two inches in thickness, and which is bored in all directions by 

 tubicolous annelids, and has its surface thus pierced all over by 



* j. W TUiclianan, " Proc. R. Soc ," No. 170, 1876, p. 613. 



