File:The Quarterly journal of the Geological Society of London (13960399383).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13960399383).jpg |
COLLECTED IN MADAGASCAR BY REV. R. BARON. 355 phyritic crystals. The presence of hornblende in the original magma is marked by pseudomorphs of iron-ore. Andesitic lavas occur at Andranonatoha and on a hill in the volcanic region to the west of Lake Itasy. They are of a darker colour than the trachytes, namely, greyish brown. They contain striated felspar, which generally gives lath-shaped sections, but also occurs in large irregular masses, imbedded in a ground-mass com- posed of prisms of yellowish-green augite, granules of magnetite, and microlites of felspar. A glassy base does not appear to be present, unless it be as extremely thin films between the felspars of the ground-mass. Large porphyritic crystals of hornblende floated in the molten magma when it was first erupted: but they have been converted into granules of iron-ore; and only small fragments of the original hornblende-substance remain to mark its former presence. To summarize briefly, we have found the older crystalline series of Madagascar, as represented by Mr. Baron's collection, to consist of foliated rocks (which are described as granitite-gneiss and tonalite- gneiss), and rocks in which no trace of foliation can be detected, comprising granite, olivine-norite, pyroxene-granulite, and pyroxenite. The majority of the granites are of the granitite type, but true granite (i.e. granite with tivo micas) also occurs. The basic members of the unfoliated series are interesting on account of their striking mineral combinations. The clear aspect and bright colouring of their con- stituent grains often make them objects of surpassing beauty when viewed under the microscope. Of deeper interest is the fact that these basic types, so well known in other areas of crystalline schists, — in Saxony, Brittany, Scandinavia, Scotland, and on the Hudson River — constitute in Madagascar, as at Kilamanjaro on the adjacent mainland, so large a part of the ancient floor on which the sedimentary rocks were laid down and through which the volcanic lavas were erupted. The volcanic rocks consist mainly of basaltic types, only a few specimens of trachyte and andesite being represented in Mr. Baron's collection. The basalts vary, as regards composition, with respect to the presence or absence of quartz, olivine, porphyritic and micro- litic hornblende, and biotite. One curious type contains idio- morphic crystals of hornblende as a constituent of the ground- mass. A felspar-free variety of magma-basalt is also represented. This rock contains only a small quantity of olivine, and is therefore intermediate in composition between the limburgite of Kosenbusch and the augitite of Dolter. |
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Date | |||
Source | https://www.flickr.com/photos/biodivlibrary/13960399383 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36940168 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 353 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36940168 | ||
Page type InfoField | Text | ||
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Flickr posted date InfoField | 21 April 2014 | ||
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26 August 2015
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current | 05:44, 26 August 2015 | 1,166 × 2,104 (518 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13960399383 | description = COLLECTED IN MADAGASCAR BY REV. R. BARON. 355 <b... |
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