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| Keywords: The Quarterly journal of the Geological Society of London (12736571605).jpg ASSOCIATED ROCKS OF THE LIZARD DISTRICT 907 <br> red variety ; for towards the outside it exhibits perfectly the change <br> into the red variety I have specimens about 4 inches long red at <br> one end and green at the other Hence the alteration may not be <br> so deep-seated as from its uniformity I should have supposed <br> I have examined three slides of the red cut from slightly different <br> varieties of the rock and one of the green I will refer to them by <br> numbers taking the most normal specimen first <br> On placing slide I beneath the microscope we find it to consist of <br> colourless or nearly colourless felspar in rather irregular to roundish <br> oblong grains occasionally showing lines indicative of twinning <br> traversed often by cracks and in places partly kaolinized Associated <br> with this in about equal quantities are a large number of irregular <br> grains of olivine ; these in parts are almost unaltered in other <br> parts entirely converted into serpentine This is occasionally <br> translucent and of a greenish yellow occasionally opaque from the <br> presence of a muddy brown peroxide of iron and showing every <br> grade of intermediate staining The process of conversion of the <br> olivine into serpentine will be described below <br> Fig 6 ” Shore below Coverack Cove <br> b ES3 <br> Newer gabbro intrusive in older gabbro and both cut by dyke of dark trap <br> a Older gabbro b Newer gabbro c Dark trap <br> With polarized light crossed prisms the felspar is seen to be <br> crystallized in irregular grains many of which show characteristic <br> plagioclase twinning ; bright colours however are rare shades of <br> light and dark milky grey being commonest In parts the felspar <br> is almost opaque from decomposition ; i i other parts it presents the <br> usual finely granular saussuritic aspect The olivine when un- <br> changed shows its characteristic rich colouring The process of con- <br> version into serpentine best examined by rotating the polarizer <br> is as follows see figs 8 9 In the cracks of the olivine a dark <br> ferruginous stain is deposited ; then on either side of this a layer of <br> fibrous serpentine of pale golden colour probably chrysotile is <br> formed ; thus the olivine grains seem traversed by an irregular net- <br> work of associated dark and light strings The interspaces then <br> seem to be attacked ; and they also are converted into serpentine ; <br> but in them the mineral is usually in an isotropic or noncrystalline <br> state and the peroxide of iron either forms a dark clot in the middle 35819102 110705 51125 Page 907 Text 33 http //www biodiversitylibrary org/page/35819102 1877 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 33 1877 Geology Periodicals Smithsonian Libraries bhl page 35819102 dc identifier http //biodiversitylibrary org/page/35819102 smithsonian libraries Information field Flickr posted date ISOdate 2014-02-24 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/12736571605 2015-08-26 19 59 32 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1877 Photos uploaded from Flickr by Fæ using a script | ||||
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