File:The Quarterly journal of the Geological Society of London (13937220822).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13937220822).jpg |
352 DE. F. H. HATCH ON THE CHARACTERS OF ROCKS sections present the cross- jointing parallel to the basal plane and the central accumulation of inclusions in the direction of the vertical axis, characteristic for apatite. The mineralogical composition of the rock is that of an augite-andesite ; but its structure is basaltic, the felspar being confined to the ground-mass, while the augite is porphyritic. With the exception of the absence of olivine it differs in no essential respect from the basalt described in section b. d. Basalt with Hornblende as a Constituent of the Ground-mass. The rock placed under this head is a basalt of rather unusual type. It occurs in one of a series of dykes on the sea-coast at Am- bodimadiro, on the north-west coast. Originally vesicular, its cavities have become infilled with fibrous zeolites and calcite. The powdered rock effervesces strongly with hydrochloric acid ; and on standing in the cold, gelatinous silica separates. No cubes of salt being formed in the solution when evaporated, the zeolites must be lime-zeolites (scolecite, phillipsite). The chief porphyritic constituent of the rock is augite in nu- merous large and well-shaped crystals, which are partially invaded by serpentinous and calcareous alteration-products. Plagioclase also occurs porphyritically ; but its crystals are few and small. The ground-mass is a plexus of felspar-laths, between which are scattered numerous small crystals and needles of brown hornblende and granules of magnetic iron-ore. Interstitial glassy matter could not be detected. In its general character the rock much resembles a tephrite : and an attempt was made to prove the presence of nepheline. But the fact that the hydrochloric-acid solution of the powdered rock gave no cubes of salt, tends to negative this supposition. After etching and treatment with fuchsine, however, a number of small specks retained the colour ; and it is quite possible that these may be granules of nepheline. It is an interesting fact with regard to this rock that, while augite appears as a porphyritic constituent, hornblende is present only in the ground-mass, thus reversing the usual order of things. The occurrence of hornblende as a constituent of the ground-mass of basalts has been mentioned by C. Chelius *. e. Felspar-free Basalt (Magma-basalt, Boiicky). Belonging to this class is a black, compact, semi-vitreous rock, from a lava-stream near the village of Ambohipolo, in the volcanic region to the west of Lake Itasy. In thin section this rock appears as a clear brown glass, containing numerous microlites and crystals of augite and magnetite, the latter in square and lozenge-shaped sections, varying in diameter from -006 mm. to -086 mm. Olivine is rare. The augite is pale ' Erlautervmgen zu Blatt Messel und Blatt Eossclorf der geolog. Karte des Grossk. Hessen.' Darmstadt, 1886. Eosenbusch, Die massigen Gest. vol. ii. p.711. |
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Source | https://www.flickr.com/photos/biodivlibrary/13937220822 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36940165 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 350 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36940165 | ||
Page type InfoField | Text | ||
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Flickr posted date InfoField | 21 April 2014 | ||
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current | 05:44, 26 August 2015 | 1,154 × 2,105 (502 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13937220822 | description = 352 DE. F. H. HATCH ON THE CHARACTERS OF ROCK... |
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