File:The Quarterly journal of the Geological Society of London (13937226491).jpg
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[edit]DescriptionThe Quarterly journal of the Geological Society of London (13937226491).jpg |
350 I)K. E. H. HATCH ON THE CHAEACTEES OE EOCKS The crystals almost invariably show zonal structure, being composed of successive isomorphous layers, in which there is a gradually in- creasing difference in chemical composition, directly influencing the position of the axes of elasticity: the extinction between crossed nicols is consequently not uniform ; it takes the form of a dark shadow, which on rotation of the section moves from the centre outwards towards the periphery. In one instance a difference of as much as 20° was measured between the peripheral layer and the central portion. Even in ordinary light a difference between the peripheral and central portions can sometimes be distinguished, the former being of a deeper tint than the latter. In sections parallel to the vertical axis the central portion often simulates the shape of an hour-glass (see fig. 6). This structure has been explained in the following way : — During the consolidation of the rock the augite first separates in skeleton-crystals of the shape of an hour-glass. Later on the depressions in the sides of these become filled in by augitic material of a slightly different composition and possessing, in consequence, an optical character deviating from that of the first-formed central portion. A curious instance of zoning in augite is shown in fig. 8. Twinning parallel to the orthopinacoid Fig. 6. — Hour-glass Fig. 7.— Augite with structure in Augite. glass-inclusion. Fig. 8. — Zoned Augite. i r is of frequent occurrence. In some cases this is polysynthetic, as in triclinic felspar (see fig. 9) ; in one case two twins are united along a common face (fig. 10). Among the earliest minerals to separate were the hornblende and mica. Of these minerals, however, the major portion has undergone resorption into the molten magma during the later stages of consoli- dation. In some cases there remain small fragments, encircled by a broad zone of the products of fusion .Schmelzzone). In the case of the hornblende this secondary material usually retains perfectly the original form of the crystals after which it is pseudo- morphous. It consists of: 1, opaque particles (magnetite); 2,doubly |
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Source | https://www.flickr.com/photos/biodivlibrary/13937226491 | ||
Author | Geological Society of London | ||
Full title InfoField | The Quarterly journal of the Geological Society of London. | ||
Page ID InfoField | 36940163 | ||
Item ID InfoField | 113696 (Find related Wikimedia Commons images) | ||
Title ID InfoField | 51125 | ||
Page numbers InfoField | Page 348 | ||
BHL Page URL InfoField | https://www.biodiversitylibrary.org/page/36940163 | ||
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 | 1,154 × 2,105 (460 KB) | Fæ (talk | contribs) | == {{int:filedesc}} == {{BHL | title = The Quarterly journal of the Geological Society of London. | source = http://www.flickr.com/photos/biodivlibrary/13937226491 | description = 350 <br> I)K. E. H. HATCH ON THE CHAEACTEES OE... |
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