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| Keywords: The Quarterly journal of the Geological Society of London (13937104882).jpg NODULAR FELSTONES OF THE LLEYN 251 <br> marked at parts to the N W and the N E by light- and dark-grey <br> lines bending around pinkish crystals and the rock under the <br> microscope is seen to be much broken and silicified Southwards <br> the mass is compact and uniform ; it shows at one place curvitabular <br> jointing similar to that figured by Professor Bonney from the <br> basalt of La Prudelle <br> II Some Petrological Characters of the Rocks <br> The general character of the ground -mass has been indicated as <br> fluidal and sometimes slaggy with a finer or coarser devitrification <br> The distinct crystals are many of them alteration-products such as <br> forms of viridite or of chlorite sometimes vermicular or spherulitic <br> Leucoxene is found often with cuneiform magnetite within and in <br> one rock some apatite Epidote is not uncommon and in several <br> slides is connected with aggregations of dark grains which although <br> obscure I believe to be themselves mainly epidote either skeleton- <br> crystals or formed as pseudomorphs possibly after augite The <br> spaces between the grains cross rectangularly as might happen <br> from a corrosion of that mineral There is some free quartz but <br> the felspars form the chief of the porphyritic crystals A few of <br> these exhibit under polarization deep purple and red colours ; some <br> are kaolinized some are clear possibly a result of pseudomorphism ; <br> and others have undergone replacements which I will discuss <br> later <br> The normal perlitic structure of glassy rocks is well shown f <br> Sometimes it is marked by viridite or a ferruginous deposit ; and in <br> slides from some noduliferous localities fibrous chalcedony stretches <br> across the cracks and fills them up resembling in miniature the <br> chalcedonic bands in the crust of many agate nodules <br> Spherulites showing the black cross occur sometimes isolated and <br> rounded J Some have viridite or a ferruginous deposit aggregated <br> towards the centre Others are clear and homogeneous and look as if <br> silicified ; these are most marked where chalcedony has been deposited <br> in perlitic cracks They often form the whole ground-mass inter- <br> locking with irregular outlines as described by Professor Bonney § <br> and doubtless indicate as he suggests some secondary change They <br> are similar to the spherulites herissees d'asperites developed <br> in glass by Daubree's experiments In some slides the spherulites <br> are not without relation to perlitic cracks which may encircle them <br> or may be directed towards the interior and be traceable again <br> beyond the interruption In one example the ground shows perlitic <br> On Columnar Fissile and Spheroidal Structure Quart Journ Geol <br> Soc vol xxxii 1876 p 146 fig 6 <br> t Compare Mr Rutley Lavas of Glyder Fawr Quart Journ Geol Soc <br> 1879 vol xxxv p 508 Rocks from Beddgelert and Snowdon Quart <br> Journ Geol Soc 1881 vol xxxvii p 403 <br> X ' British Petrography ' J J H Teall pi 38 <br> § Pres Addr Geol Soc 1885 p 68 <br> Geol Exper p 170 fig 45 ; see also a group in the Saulieu rock Min <br> Micr Fouque and Levy pi xii fig 1 36940044 113696 51125 Page 249 Text 45 http //www biodiversitylibrary org/page/36940044 1889 Geological Society of London NameFound Spherulites NameConfirmed Spherulites NameBankID 4348532 Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 45 1889 Geology Periodicals Smithsonian Libraries bhl page 36940044 dc identifier http //biodiversitylibrary org/page/36940044 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 N02/13937104882 2015-08-26 05 56 20 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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