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| Keywords: The Quarterly journal of the Geological Society of London (13937312691).jpg 484 DK C CALLAWAY ON SECONDAKY MINEEALS IN THE <br> must be regarded as the effect of secondary action This hornblende <br> compared with that of the preceding slides is darker brighter <br> clearer and more strongly pleochroic Chlorite sometimes appears <br> in the cleavage-planes of the hornblende and increases in quantity <br> at some points so as to form nests of rosettes irregular blotches or <br> patches running with the foliation Under crossed nicols it gives a <br> deep indigo -colour passing into grey In one of the slides there is <br> a fair quantity of epidote <br> Much of the felspar is clear and sound and a large proportion of <br> it shows the twinning of plagioclase ; but some of it has begun to <br> break up into an indistinct mosaic and displays numerous microlitbs <br> of a clear mineral white mica It is important to note that <br> this felspar is moulded upon all the preceding minerals and even <br> includes shreds of the chlorite and minute fragments of broken <br> hornblende- crystals The chlorite is certainly derived from the <br> hornblende and the distortion and fracture of the hornblende is a <br> secondary result produced by pressure subsequent to consolidation <br> since as I have stated it is frequently connected with the formation <br> of planes of discontinuity That this felspar is secondary receives <br> some confirmation from its clear and fresh appearance <br> The slides contain a small proportion of granular quartz some of <br> which contains liquid-cavities with air-bubbles <br> No 314 is taken from within a few inches of the veined plexus <br> Much of the hornblende is dragged out in the direction of foliation <br> and the whole of it is more or less dirty and opaque The opacity is <br> apparently due to liberated ferric oxide Most of the felspar is <br> either cloudy or contains countless clear microliths white mica <br> and in some there are patches of white mica There is also a great <br> quantity of finely granular almost opaque matter in the felspar <br> It may be epidote or zoisite Granules of quartz are scattered through <br> the slide <br> No 391 About the same distance from the zone but a little <br> higher up The hornblende is like the last but the curvilinear <br> outlines may be sometimes made out The felspar is also similar <br> The proportion of granular quartz is greater The noteworthy <br> feature of the slide is the coming-in of black mica It occurs in <br> elongated flakes several laminae in thickness and pointed at each end <br> They run parallel to the general foliation often passing through the <br> middle of hornblende-crystals and are occasionally associated with <br> a little chlorite The habit of these flakes is precisely that of the <br> chlorite which forms in the interior of the hornblende and frequently <br> projects beyond it The felspar is clearly moulded upon the mica <br> and sometimes completely encloses it <br> No 316 is in contact with the shear-zone The rock is rather <br> more compressed than in the last two slides The hornblende is <br> similar It sometimes passes into chlorite which becomes banded <br> here and there with black mica or is entirely replaced by it <br> Sometimes a little opaque flake probably ferruginized hornblende <br> projects from a bit of hornblende into a patch of chlorite and just <br> round the margin of the flake black mica is formed showing marked 36940311 113696 51125 Page 482 Text 45 http //www biodiversitylibrary org/page/36940311 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 36940311 dc identifier http //biodiversitylibrary org/page/36940311 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 N02/13937312691 2015-08-26 05 52 00 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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