. British Antarctic expedition, 1907-9, under the command of Shackleton : reports on the scientific investigations ; geology. Scientific expeditions; Natural history; Geology; Paleontology; Petrology; Ice. Fig. la Fig. lb minerals, but cannot be considered a phenocryst, since it includes abundantly the fine hornblende needles of the ground-mass. It shows simple albite twins with low extinction angles, but too few opportunities of measurement present themselves to determine the maximum, nor can the refringence be compared with that of balsam since the mineral is always surrounded by analci
. British Antarctic expedition, 1907-9, under the command of Shackleton : reports on the scientific investigations ; geology. Scientific expeditions; Natural history; Geology; Paleontology; Petrology; Ice. Fig. la Fig. lb minerals, but cannot be considered a phenocryst, since it includes abundantly the fine hornblende needles of the ground-mass. It shows simple albite twins with low extinction angles, but too few opportunities of measurement present themselves to determine the maximum, nor can the refringence be compared with that of balsam since the mineral is always surrounded by analcite. The latter occupies large areas of the ground-mass, of which the chief constituent is hornblende in small prisms crossing one another in all directions. The amygdules are generally completely filled with analcite and calcite, the former being the earlier mineral in sharp crystals (Fig. 6, PL II). On the assumption that a calcic felspar has been replaced by analcite and calcite, the rocks agree in all respects with olivine camptonites. Origin of the Hornblendic Inclusions of the Trachytes Lacroix has found that inclusions more basic than their host, although not so common as inclusions of sanidinites, have nevertheless a wide distribution in trachytes, especially in those rich in ferromagnesian minerals. They are for the most part of the nature of porphyrites. In phonolites the analogous inclusions are generally camptonites, rocks which, he points out, frequently accompany nepheline syenites, the plutonic representatives of phonolites. He considers them, nevertheless, as basic segregations from the phonolitic magma.* While the Observation Hill inclusions might easily be considered as segregations, since the same hornblende is so abundant in the trachyte, the Cape Bird camptonites cannot be so regarded. The alteration they have undergone could scarcely have taken place had they never existed but within the trachyte, since the latter is a relatively fresh rock. It is simpler t
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Keywords: ., boo, bookcentury1900, booksubjectgeology, booksubjectpaleontology
