Report on the scientific results of the voyage of H.M.SChallenger during the years 1873-76 : under the command of Captain George SNares, R.N., F.R.Sand Captain Frank Turle Thomson, R.N. . d, and contains tabular crystals ofsanidine measuring 3 to 5 mm. Microscopic sections show a ground-mass composed oflamellae of tridymite with a faint yellow colour. Bather large sections of felspar andaugite can be distinguished in it; this latter mineral is frequent in small sectionsembedded in a tridymite mass. Two kinds of felspar are to be seen; some lamellaehave small extinctions like those of oligoclas

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Report on the scientific results of the voyage of H.M.SChallenger during the years 1873-76 : under the command of Captain George SNares, R.N., F.R.Sand Captain Frank Turle Thomson, R.N. . d, and contains tabular crystals ofsanidine measuring 3 to 5 mm. Microscopic sections show a ground-mass composed oflamellae of tridymite with a faint yellow colour. Bather large sections of felspar andaugite can be distinguished in it; this latter mineral is frequent in small sectionsembedded in a tridymite mass. Two kinds of felspar are to be seen; some lamellaehave small extinctions like those of oligoclase, which is known to occur in the olderrocks containing orthoclase and quartz. The other felspathic sections are those of amonoclinic felspar; they have irregular and indistinct outlines, and never show poly-synthetic striation, but they are twinned and composed of two individuals. The outlinesof these sections and their extinction show that this felspar is twinned according to thelaw of Manebach; these sections show, like sanidine twinned according to the Carlsbadlaw, two halves joined together, but, whereas in a Carlsbad twin, the direction of 12 THE VOYAGE OF H.M.S. CHALLENGER.. Fig. 5.—Altered rock below Casa Blanca. Section of sanidine twinned accordingto 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 linesof cleavage belongs to the trace of P (composition plane). The other, less marked, isthe prismatic cleavage. The two halves of the sections extinguish symmetrically at an angle of about 7° {A A), the extinction beingpositive (see fig. 5). These details show that this mineralis sanidine. In some cases it has crystallised according tothe Carlsbad law. The augitic sections are greenish, and they are not very common. The ground-mass con-tains augite microlith