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



work of cleavages appears, crossing at angles of about 87° ; the extinction on the face 

 oogoo is more than 35°. The position of the optic axis being in the plane of symmetry, 

 this mineral cannot be mistaken for a rhombic pyroxene ; while, if the phenomena of 

 pleochroism only were to be taken into account, there would be no hesitation in viewing 

 these sections as allied to hypersthene, all the more because, like the latter mineral, 

 they have a certain fibrous structure. It is very probable that this monoclinic 

 pyroxene has often been confounded with hypersthene, but in the present case, the 

 angle of extinction, and the phenomena in convergent light, make the determination as 

 augite craite certain. The intense pleochroism is — 



/S > y = a 



reddish. sea-green. 



Hornblende in large greenish sections is much more widely diffused through the rock 

 than augite, and it is only formed at the expense of the latter. In examining more 

 minutely the relations connecting these two minerals, we observe phenomena of 

 alteration and pseudomorphism, more magnificent examples of which than those of 

 the Falkland Islands it would be hard to find. Augite grains can rarely be seen 

 without a surrounding zone of greenish amphibolic matter. Decomposition commences 

 in the microscopic fissures which furrow the surface of the augite ; these become covered 

 with a yellowish coating, making them clearly visible. If the optical properties 

 were not taken into account, one might confound the augite, altered in this way and 

 surrounded by the secondary product, with some sections of decomposed olivine. The 

 colour and relief are the same, and the roughened surface and products of alteration 

 present the same microscopic appearances in the two minerals. At a more advanced stage 

 of decomposition the fissures appear wider, the secondary product spreads out, sometimes 

 entirely surrounding a nucleus of nearly unaltered augite. The mineral formed in this 

 way at the expense of the augite passes from its yellowish colour to green, takes on a 

 finely fibrous texture at the place of contact with augite, becomes filled with opaque, 

 blackish, ferruginous grains, and unites laterally with patches of clearly characterised 

 hornblende. These, as we have said, always surround a fragment of augite, which 

 remains as a nucleus in the middle of the hornblende. 



The hornblende appears in large yellowish brown sections, with the optical 

 characters and cleavage of this species, but never surrounded by crystallographic 

 contours. The large amphibolic patches are moulded on the neighbouring minerals, 

 and do not present the more or less prismatic form which augite preserves in spite of 

 the granular texture of the rock. In a word, the characters of the hornblende mark 

 it out as having been formed after all the other minerals in the rock, and its relation 

 to augite shows that it has developed from the latter. "We have thus a perfectly 

 clear case of amphibolisation of pyroxene. It is interesting, besides, to note that 



