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| Keywords: The Quarterly journal of the Geological Society of London (12733784715).jpg J W JUDD ON THE SECONDARY ROCKS OF SCOTLAND <br> 279 <br> less regular form while they often extend to much greater length <br> than those of the latter class unlike which again they are never di- <br> vided into regular blocks by equidistant curved joint-planes One <br> of the best examples of the columnar structure among the old lavas <br> Fig 8 ” Cliff-section 8 W of Oban exhibiting very fine Columnar <br> Structure in the Porphyrite Lavas of Lorn <br> of Lorn is that exhibited in a cliff about two miles south-west of <br> Oban where the columns are of great length and beautifully curved <br> see woodcut fig 8 ; but more or less perfect examples of the same <br> structure are exposed on the face of Beinn Lora and at other points <br> Some of the volcanic rocks of Lorn present characters which at <br> first sight appear not a little anomalous and puzzling ; but this is a <br> circumstance which when we remember the metamorphic processes <br> to which lavas in common with all other rocks are subject need <br> scarcely occasion surprise Certain of the lavas appear after their <br> emission to have weathered into the characteristic spheroidal and <br> concentric forms while others have evidently decomposed into a <br> wackose condition ; and in both cases the rocks which result from <br> the re-induration of such masses present very peculiar features and <br> may in some instances be mistaken for consolidated trap-tuffs <br> The separate lava-flows were often of enormous thickness As a <br> general rule the great body of each of these streams is made up of a' <br> compact often highly porphyritic rock ; but towards the upper and <br> under surfaces of the mass it usually assumes the amygdaloidal <br> structure The amygdaloidal cavities which have evidently served <br> as chemical laboratories in which very complex operations have been <br> carried on are frequently deprived of their contents by recent wea- <br> thering operations ; and the original structure of the rocks is thus to a <br> great extent restored Then is made clearly apparent the originally <br> highly vesicular character of the upper and under portions of these an- <br> cient lava streams the vesicles being often seen to be drawn out in the <br> x2 35766174 110599 51125 Page 278 Text v 30 http //www biodiversitylibrary org/page/35766174 1874 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 30 1874 Geology Periodicals Smithsonian Libraries bhl page 35766174 dc identifier http //biodiversitylibrary org/page/35766174 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 N02/12733784715 2015-08-26 20 05 01 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1874 Photos uploaded from Flickr by Fæ using a script | ||||
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