NARRATIVE OF THE CRUISE, 207 



The structure is microgranitoid ; rarely the sections of olivine or enstatite assume dimensions large 

 enough to produce a microporphyritic structure, which passes into a banded structure, the minerals 

 constituting thi3 rock never have crystallographic contours, but are elliptical or irregular. This 

 feature and the banded structure give rise to a sort of lenticular arrangement, which resembles 

 the so-called gneissic structure peculiar to some schists. Without entering on a detailed description 

 of the individual minerals that constitute the rock, it may be stated that the microscopic examination 

 of the specimens shows that the rock mass is almost entirely composed of granular olivine, thus 

 confirming the deductions drawn from the chemical analysis. 1 After the olivine the most frequent 

 ingredient is chromite; the sections of this mineral are generally transparent yellow or chestnut-brown 

 and isotropic. Among the minerals playing a secondary part in the composition are hornblende and a 

 rhombic pyroxene. The hornblendic mineral must be referred to the variety actinolite, of which it 

 seems to possess the most characteristic properties ; the rhombic pyroxene, on the other hand, must 

 be classed as enstatite. These ellipsoidal sections of enstatite are polysynthetic, and composed of 

 lamellae of a rhombic pyroxene, between which are intercalated other lamella? of a cliuorhombic 

 pyroxene. 



" Certain features of the olivine, and more especially those shown in the enstatite sections, 

 deserve attention. In some microscopic preparations of the rocks, with banded structure, the larger 

 sections of olivine and enstatite are placed with their vertical axes in a line with the direction of 

 the bands. At first sight it looks as if this disposition had been brought about by the motion of a 

 plastic mass. In one case, where the fragments were in the direction of the band, a crystal has under- 

 gone a remarkable process of folding or curling back upon itself by fracture and displacement, 

 it seems to have been partially softened, and looks as if a current had drawn it along and bent 

 it into the shape of a U. The lamella? composing the crystal are fractured at the summit of the 

 arch of the curve, and the space between the fractures is filled up with the ground mass of the rock. 

 Sections presenting the same appearance may, however, be found abundantly in the family 

 of the schists. Among the analogies of microscopic structure between the schists and the peridote 

 of St. Paul's Rocks may be enumerated the ellipsoidal form of the crystals, their entwinement by 

 the bands in the fundamental mass, the disruption of the larger individuals, as well as their curvature 

 and folding. 



" Some of the specimens are highly altered. Along the capillary fissures cohesion diminishes, and 

 serpentinous matter with magnetic iron is deposited in them, the rock being traversed at the same 

 time by black, opaque, and slightly lustrous veins. These altered specimens are often composed of 

 fragments of serpentine cemented together by phosphate of lime, which also often coats the external 

 part of the rock, and to this circumstance these altered portions owe a particular stalactitic appearance. 

 The white enamel that gives the south rock the dazzling appearance described by Darwin, was 

 removed and subjected to a quantitative analysis. The quantity analysed (0-0175 gramme) was so 

 minute, that the only certain results obtained were phosphoric acid, 3361 per cent, and lime 50 - 51 per 

 cent. ; iron, magnesia, and sulphuric acid were also present. The composition is, therefore, essentially 

 a tribasic calcic phosphate, with sulphate of lime, and perhaps also carbonate of lime, magnesia, and 

 iron. Darwin and Mr. Buchanan regard this white coating as due to the accumulation of excrement 

 of sea birds, the insoluble residue of which has been exposed d.uring very long periods of time to the 

 action of the sun's rays and of the waves of the ocean. This explanation seems the true one, and is 



1 For the mineralogical description of the Rocks, see Narr. Chall. Exp., vol. ii., App. B. 



2 Ibid., fig. 2 of the plate. 



