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



Some of the specimens, as already mentioned, have an exceedingly fresh and unaltered 

 appearance ; many, on the contrary, are deeply altered. I will now describe these products 

 of decomposition, and the fragments covered or impregnated with phosphate of lime. 



The specimens showing the fewest traces of alteration are greenish-grey in colour, with 

 a vitreous lustre ; thin sections prepared from them show the ground-mass to be perfectly 

 granular. Along the capillary fissures cohesion diminishes, and a blackish secondary 

 product is deposited between the granules or the more coherent portions. This may be 

 called the first stage of alteration ; the more advanced stages are marked by a widening 

 of the fissures, and by the presence of a transparent, fibrous, greenish, or slightly yellow 

 serpentinous substance, partly isotropic, with irregular black grains that must be classed 

 as magnetite. It is unnecessary to describe here the alteration of olivine into serpentine, 

 which has already been done so thoroughly by many microscopists. We may pass on to the 

 description of the specimens which have undergone most alteration. In these the ground- 

 mass, instead of being compact, with deep lustrous tints, is almost brittle, of a greenish- 

 yellow hue, and crossed in all directions by black opaque and slightly lustrous veins. 

 These specimens have the characteristics of serpentine. In the less altered specimens, 

 black veins run along the regular fissure-lines already described. All these marks show 

 serpentinous alteration-products. The sections resemble breccias, and have their joints 

 completely impregnated with phosphate of lime. A specimen is occasionally found 

 containing fragments of unaltered olivine ; but, as a rule, most of the specimens are changed 

 into a greenish-yellow serpentine with dark veins. In a more advanced stage of decom- 

 position, the small dark veins stand out in relief, the surface of the serpentine being softer, 

 disintegrates and falls away, thereby giving the surface of the rock a honeycombed look. 

 The projecting veins are not infrequently covered with a deposit of mamillated phosphate 

 of lime ; concretions of the same substance fill the small honeycomb-cells, and all the joints 

 are covered with a light coating of it. 



To this circumstance these altered portions of the rock owe their peculiar stalactitic 

 appearance in which they so resemble certain varieties of ores of zinc, or some phos- 

 phorites. Some of the specimens are composed of fragments of serpentine cemented 

 together by veins of phosphate of lime 5 to 6 millimetres broad. These veins are compact, 

 and to the naked eye, or, better still, under the pocket-lens, they show an indistinct concre- 

 tionary structure, the more or less circular forms of which present yellowish or rosy-white 

 zonal bands, like some of the ribboned agates. Between concretions are often found 

 cavities that give the whole mass an appearance not unlike that of the cherts of the 

 Carboniferous limestone, in which the cavities are formed by the disappearance of 

 calcareous fossils. The compactness of grain and the other macroscopic characters might, 

 at first sight, readily cause the concretions to be taken for fragments of chert. 1 Their 

 decomposition results in a fine pinkish-white powder. 



1 Darwin is probably speaking of tins phosphate when lie remarks : " In some parts the rock is of a cherty nature " 

 (loc. cit., p. 8). 



