. British Antarctic expedition, 1907-9, under the command of Shackleton : reports on the scientific investigations ; geology. Scientific expeditions; Natural history; Geology; Paleontology; Petrology; Ice. 170 PETROLOGICAL NOTES ON SOME OF THE ERRATICS Microscopic Characters.—The rock is hypidiomorphic granular in fabric with a medium grainsize. Felspar makes up about 80 per cent, of the minerals present. Plagioclase about Ab5An3, is present in subidiomorphic sections up to 1 mm. dia- meter. It shows twinning after the albite Carlsbad and Manebach laws, the striations being very fine and
. British Antarctic expedition, 1907-9, under the command of Shackleton : reports on the scientific investigations ; geology. Scientific expeditions; Natural history; Geology; Paleontology; Petrology; Ice. 170 PETROLOGICAL NOTES ON SOME OF THE ERRATICS Microscopic Characters.—The rock is hypidiomorphic granular in fabric with a medium grainsize. Felspar makes up about 80 per cent, of the minerals present. Plagioclase about Ab5An3, is present in subidiomorphic sections up to 1 mm. dia- meter. It shows twinning after the albite Carlsbad and Manebach laws, the striations being very fine and showing pretty examples of bending. The mineral is somewhat decomposed, particularly near the periphery. Orthoclase is untwinned, slightly decomposed, and exhibits shadowy extinction. It is moulded on plagioclase, and in some cases there is apparently a crystallographic relation between the two, the outgrowths of orthoclase showing optical continuity Quartz is not very allotriomorphic, and shows shadowy extinction due to strain. Biotite calls for no special description, it is somewhat frayed and contains inclusions of apatite. There is also a very little light-coloured Fig. 1. Sketch showing Kelation of Plagioclase and Orthoclase. The grains of orthoclase indicated by X are optically continuous. 439. Aplite Microscopic Characters.—Allotriomorphic granular rock with an average grainsize of 0 '5 mm., consists essentially of quartz and orthoclase with less abundant albite and very little decomposed biotite. Quartz shows slight evidences of strain; orthoclase is quite ordinary. The plagioclase present is between albite and oligoclase in composition. Rocks of this type are extremely abundant amongst the erratics; a fact attribu- table to their resistance to weathering being greater, as a rule, than that of the normal biotite or hornblende granites with which they were in all probability genetically Please note that these images are extracted from scanned pa
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Keywords: ., boo, bookcentury1900, booksubjectgeology, booksubjectpaleontology
