12 



THE VOYAGE OF H.M.S. CHALLENGER. 



Fig. 5. — Altered rock below Casa Blanca. 



Section of sanidine twinned according 

 to the Manebach law. 



cleavage remains the same for both individuals in the section, with the twin of Mane- 

 bach each individual has its cleavages, ending at the line of the composition plane, 

 forming an angle of about 66° with one another. One of the two better marked lines 

 of cleavage belongs to the trace of P (composition plane). The other, less marked, is 

 the prismatic cleavage. The two halves of the sections extinguish symmetrically 



at an angle of about 7° {A A'), the extinction being 

 positive (see fig. 5). These details show that this mineral 

 is sanidine. In some cases it has crystallised according to 

 the Carlsbad law. The augitic sections are greenish, 

 and they are not very common. The ground-mass con- 

 tains augite microliths embedded in lamellae of tridymite. 

 Under low magnifying powers it might be fancied that 

 the rock possesses perlitic structure or contains trichites, 

 but under higher powers it is ascertained that these 

 indistinct forms and lines are extremely thin lamellae, 

 superposed one upon the other or imbricated as in the case 

 of tridymite. The hexagonal outlines of these lamellae 

 are shown by rather distinct traces, rendered slightly 

 more apparent by a brownish coloration due to limonite. 

 This fact is analogous to what is often observed for tridymite in other eruptive rocks, 

 and in some meteorites. Generally the scales in cpiestion are well outlined ; in other 

 cases they are, as it were, slightly notched. Their optical properties cannot be studied 

 on account of their extreme thinness and their superposition. All that can be said is 

 that the colours of polarisation are faint, and similar to those of quartz in sections of 

 the same thickness as that of the tridymite. 



Other specimens collected on the same excursion to the Peak are augite-andesites, 

 more or less scoriaceous, and felspathic basalts with or without vitreous base, often 

 globulitically devitrified. These rocks do not present any character which was not 

 mentioned in the basalts described above. Several specimens of obsidian were also 

 collected, with alternate black and greyish bands, often more or less fibrous, on 

 account of the elongation of the pores. A striped and fibrous obsidian exactly 

 resembles pumice, except that in the former there are massive portions. These 

 obsidians are rich in trichites of various forms, which are more numerous the fewer 

 minerals the rock contains. Among the latter may be noticed plagioclase, hornblende, 

 augite, and magnetite. Small felspar lamellae are seen in the vitreous mass, straight 

 or slightly crescent shaped, indented at the two ends. The pumice collected does 

 not show any difference from the obsidian except in structure. It has a light greenish 

 tint, and a silky appearance. No minerals can be distinguished by the naked eye, 

 but with the microscope felspar, hornblende, augite, and magnetite can be seen. 



