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| Keywords: The Quarterly journal of the Geological Society of London (13958690233).jpg 1866 <br> JVKES OLD BED SANDSTONE AND DEVONIAN <br> 359 <br> believe the Mortehoe Simonsbath and Exton slates to be above <br> those of the Lynton district ; I believe that it is next to impossible <br> to assign a thickness to the former with anything like the approxi- <br> mation to certainty we might possibly do to the latter and that <br> their apparent may be vastly in excess of their real thickness <br> In making observations on the dip of the beds in a great section of slates <br> even the most practised observers may be occasionally led astray <br> If the beds undulate vrhile the dip of the cleavage is steady and if the sub- <br> stance of the slate be very homogeneous it may easily happen that the stratifi- <br> cation is only vrell shown when it has a given relation to the cleavage when for <br> instance they coincide or when they are at right angles to each other or when <br> they cross at some other angle so that the particular mark of stratification which <br> the kind of slate possesses shall be least obscured by the cleavage But when <br> in observing a long section a number of these comparatively well-marked dips <br> occur at several intervals all in the same direction an observer is apt to sup- <br> pose that the intermediate parts dip also in the same direction and base his <br> estimate of thickness on the width of the whole mass and its mean angle of in- <br> clination It is possible he may be right but it is also quite possible that he <br> may be wrong ; and that the section may only show the same comparatively <br> small group of beds undulating across it as suggested in fig 15 where the parts <br> showing the bedding distinctly are supposed to be those which coincide with <br> the cleavage <br> N rift- s <br> Fig 16 <br> a b c d Places where the stratification coincides with the cleavage and is there <br> observable while in the intermediate places it is not discernable or vice versa <br> If in such a section the bedding were <br> well seen at a b c and d and not at all <br> observable in the intermediate parts the <br> whole mass might be supposed to dip <br> south at 45° and show a continuous suc- <br> cession of beds instead of a repetition of <br> the same <br> While on the subject of cleavage I <br> may perhaps be allowed to mention a <br> structure I observed at Mortehoe which <br> I took at first for stratification but had <br> afterwards reason to doubt it; I have <br> often observed it before and it is some- <br> times very deceptive and may accord- <br> ingly lead to very erroneous conclusions <br> Along certain parallel bands in the slate <br> about 1 inch in width and a foot or two <br> apart there occurs a shake or crumple <br> in the cleavage-planes Viewed in sec- <br> tion it would be as in fig 16 <br> It is however when observed on the <br> face of a bed that it is most deceptive <br> as the change of tint resulting from the <br> difference in the angle of reflection of <br> the light gives to the little bands the ap- <br> pearance of a difference of colour in the <br> substance of the rock so that they might <br> be taken for the true stripe of the slate 36164800 111477 51125 Page 359 Text v 22 http //www biodiversitylibrary org/page/36164800 1866 Geological Society of London NameFound Nucula rift NameConfirmed Nucula EOLID 10719654 NameBankID 2691879 Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 22 1866 Geology Periodicals Smithsonian Libraries bhl page 36164800 dc identifier http //biodiversitylibrary org/page/36164800 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 N02/13958690233 2015-08-26 06 54 51 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1866 Photos uploaded from Flickr by Fæ using a script | ||||
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