Keywords: The Quarterly journal of the Geological Society of London (13205688485).jpg <br> SHARPE ON SLATY CLEAVAGE <br> 101 <br> disturbing agent and the connection between it and the effect it <br> has produced are obvious <br> A more difficult case is that of an elevated area which has not <br> been broken through in the middle by any great igneous eruption ; <br> where we can only infer what was the nature of the disturbing cause <br> from observing the effects it has produced This is the case of <br> the Wealden elevation which Mr Hopkins has illustrated This is <br> also the case of the elevations of Carnarvonshire and Merionethshire <br> and of Devonshire and Cornwall ; for though in all these counties <br> abundant eruptions of igneous matter have taken place some probably <br> connected with the elevation we have to consider these are secondary <br> pheenomena resulting from a wider-spreading cause ; and in Devon- <br> shire and Cornwall the great granitic eruptions do not form part of <br> the case under consideration for they were evidently hardened and <br> solidified on the surface before the elevation took place which was <br> connected with the cleavage and gave it that symmetrical arrange- <br> ment we now find <br> The hypothetical case put by Mr Hopkins of a fluid mass below <br> the surface forced upwards along a rent bounded laterally by two <br> parallel walls and covered over by the beds forming the crust of the <br> earth is in itself probable and will be found sufficient to explain <br> most of the phaenomena observed The effects produced on the <br> mass above will depend on the width of the rent relatively to the <br> fluidity of the moving mass supposing always for simplicity of ar- <br> gument that the fluid matter finds no means of escape between the <br> beds but continues always to press upwards If the rent is narrow <br> compared to the matter forced into it the surface must give way in <br> a long crack through which the fluid will escape as in the Malvern <br> Hills If the rent is broader the fluid must force up the beds above <br> it in a curve for the surfaces of the fluid mass must rise more in the <br> middle than at the sides <br> Fig 22 <br> Figure 22 represents a section of the supposed case; the lines <br> A B are the vertical walls of the fissure bounding a fluid mass which <br> can only escape upwards by displacing the mass above of which the <br> part first opposed to it is shown in its original position by the line <br> C The beds above will be successively raised into each of the <br> curves a c a since the pressure is greatest in the middle of a fluid <br> column and the resistance of the mass above is greatest at the sides 36932830 113687 51125 Page 101 Text v 3 http //www biodiversitylibrary org/page/36932830 1847 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 3 1847 Geology Periodicals Smithsonian Libraries bhl page 36932830 dc identifier http //biodiversitylibrary org/page/36932830 smithsonian libraries Information field Flickr posted date ISOdate 2014-03-17 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/13205688485 2015-08-26 12 56 07 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1847 Photos uploaded from Flickr by Fæ using a script |