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



direction, the deviation is the same for the \vhole breadth of the band. Examples of this 

 arrangement are seen wherever large sections of crystals lie in the path of the band, for 

 then the latter divides into two smaller ones, which seem to flow round the obstructing 

 crystal, and reunite on the opposite side, when the band resumes the direction and size 

 it had before. This feature gives rise to a sort of lenticular arrangement, which greatly 

 resembles the so-called gneissic structure peculiar to some schists. 



The sections of the constituent minerals are placed with their principal axes running 

 in the direction of the bands, that is, parallel with them. This feature is readily 

 perceived when they reach 1 or 2 millimetres in size. They are rounded or elliptical, 

 sometimes terminated by vague crystallographic outlines, which occasionally disappear 

 altogether, and the mineral then gradually melts away into the ground-mass and cannot 

 be distinguished from it. Professor Bonney l has observed the same structure more par- 

 ticularly in the serpentine rocks of the Lizard. This banded structure may be produced 

 by various causes, and I shall have to say something more about it on a subsequent page. 



The microscopic slices are crossed in every direction by numerous joints or cracks, 

 which seem to extend in straight lines, and intersect one another at more or less constant 

 angles, sometimes dividing the slide into rhomboidal sections. 2 These joints seldom 

 exceed O'l mm. in breadth, are filled with a greenish-yellow substance, and sometimes 

 look like black streaks (figs. 1, 2, 3). This jointed or fissured structure would appear to 

 be characteristic of rocks of this type. A great many of the slices examined under 

 the microscope are covered with a perfect network of extremely delicate fissure lines, 

 intersecting each other in every possible direction, with minute particles of a black 

 opaque substance running along both sides of the joints. Whenever these joints, 

 or crevices, are less perfectly marked, the ground-mass appears to lose its cementing 

 power along their edges, and to become more granular. When the crevices widen 

 they are commonly filled up with a yellowish serpentinous substance and black opaque 

 grains. 



I will now enter on the detailed description of the individual minerals that constitute 

 the rock. A detailed microscopical examination of the specimens shows that the rock- 

 mass is almost entirely composed of granular olivine, thus confirming the deductions 

 drawn from chemical analyses and the microscopical examinations already described. 

 Slides prepared from compact specimens, and examined with a power of 200 diameters, 

 prove the ground-mass to consist of grains of olivine, with irregular, ill-defined outlines 

 (fig. 1), sometimes slightly elongated, and sometimes blending so perfectly as to produce 

 the appearance of a homogeneous substance (fig. 3). These grains, much more minute 



1 Bonney, On the Serpentine and associated Rocks of the Lizard District, p. 920. 



2 These regular joints, so similar to cleavage, may have caused Darwin to describe fragments of this rock as 

 similar to blocks of altered felspar. The hand specimens also show these joints very distinctly when examined 

 macroscopically. 



