MAKE A MEME View Large Image The Quarterly journal of the Geological Society of London (13937219641).jpg COLLECTED IN MADAGASCAR BY REV R BARON 349 <br> The doleritic type is also represented Such a rock occurs for <br> instance at Mojanga on the north-west coast of ...
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Keywords: The Quarterly journal of the Geological Society of London (13937219641).jpg COLLECTED IN MADAGASCAR BY REV R BARON 349 <br> The doleritic type is also represented Such a rock occurs for <br> instance at Mojanga on the north-west coast of Madagascar In <br> this rock there is a marked ophitic structure large allotriomorphic <br> grains of augite being penetrated by prisms and microlites of felspar ; <br> the rock is nearly noncrystalline glassy matter being present only <br> in thin films between such felspar needles as are not enclosed in <br> the augite <br> A very curious rock which though diverging widely from the <br> normal structure of the olivine basalts may here be conveniently <br> described occurs three or four miles east of the village of Eeno- <br> arivo in the district of Yalalafotsy 60 miles west of the capital <br> It is a compact rock composed of augite felspar olivine and <br> magnetite <br> The most abundant constituent is augite This mineral occurs <br> in small sharply contoured prismatic crystals of a pale brown colour <br> Under the microscope the crystals give lath-shaped sections with <br> pyramidal terminations Twinning parallel to 100 is frequent <br> In several cases it is polysynthetic as in the triclinic felspars The <br> extinction-angle has a maximum value of about 42° <br> These little crystals of augite are imbedded in a matrix of felspar <br> The latter presents no trace of twin-striation In places it is slightly <br> kaolinized The relation between the felspar and augite may be said <br> to be ophitic ; but it differs from that structure as represented in <br> the dolerites in that in the case under consideration the felspar <br> is moulded upon idiomorphic crystals of augite instead of augite <br> upon felspar as in the dolerites Similar relations between felspar <br> and augite have been described by Rosenbusch in reference to <br> vogesites and by Teall f in certain Scotch traps <br> The order of consolidation in the Madagascar rock appears to <br> have been as follows ” <br> f Magnetite <br> 1st phase -j Olivine 2nd phase Felspar <br> Augite <br> b Olivine- Basalt with Hornblende and Biotite in PorpTiyritic <br> Crystals ffoniblende-olivine-basalt <br> A rock of this composition occurs in the volcanic region west of <br> Lake Itasy Its predominant constituent is augite occurring in <br> large black crystals imbedded in a slightly vesicular ground-mass <br> the latter being made up of microlites of felspar granules of olivine <br> augite and iron-ore together with a small quantity of interstitial <br> glass <br> Under the microscope the augite appears sometimes in irregular <br> masses sometimes in characteristic octagonal sections bounded by the <br> faces 110 100 010 Included material is abundant namely ” glass <br> magnetite olivine and more rarely a microlite of brown hornblende <br> Die massigen Gesteine 1887 p 316 <br> t British Petrography 1888 pp 188 194 214 36940162 113696 51125 Page 347 Text 45 http //www biodiversitylibrary org/page/36940162 1889 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 45 1889 Geology Periodicals Smithsonian Libraries bhl page 36940162 dc identifier http //biodiversitylibrary org/page/36940162 smithsonian libraries Information field Flickr posted date ISOdate 2014-04-21 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/13937219641 2015-08-26 05 53 26 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1889 Photos uploaded from Flickr by Fæ using a script
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