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| Keywords: The Quarterly journal of the Geological Society of London (12645678164).jpg 1849 <br> DAWSON ON THE GYPSUM OF PLAISTER COVE <br> 337 <br> shore at once recedes and the gypsum occurs at the head of the Cove <br> The gypsum is well-exposed in a cliff about 80 feet in height ; but <br> like most other large masses of this rock it is broken by weathering <br> into forms so irregular that its true dip and direction are not at first <br> sight very obvious On tracing its layers however it is found to have <br> the same dip with the subjacent limestone and marl About two-thirds <br> of the thickness of the bed consist of crystalline anhydrite and the <br> remaining third of very fine-grained common gypsum The anhy- <br> drite prevails in the lower part of the bed and common gypsum in the <br> upper ; but the greater part of the bed consists of an intimate mixture <br> of both substances the common gypsum forming a base in which mi- <br> nute crystals of anhydrite are scattered ; and bands in which anhy- <br> drite prevails alternating with others in which common gypsum pre- <br> dominates It is traversed by veins of compact gypsum but I saw <br> no red or fibrous veins like those of the marl In some parts of the <br> bed small rounded fragments of grey limestone are sparingly scattered <br> along layers of the gypsum <br> The exposed part of the mass is riddled by those singular funnel- <br> shaped holes named Plaister pits sections of which are exposed in <br> the cliff; they penetrate both the anhydrite and common gypsum <br> though they are contracted where they pass through harder portions <br> of the rock and especially the veins of compact gypsum some of which <br> are only slightly inclined and look at first sight like layers of depo- <br> sition The pits of which I saw sections have evidently resulted <br> from the percolation of water through the more open parts of vertical <br> joints and they were cut off where they were mtersected by another <br> slightly inclined set of open fissures which afforded a passage to the <br> water The accompanying sketch fig 2 shows one of these pits and <br> its relations to the joints and stratification of the gypsum <br> 6 Above the gypsum <br> 2 <br> Fig <br> Plaister Pits <br> are a few layers of limestone <br> portions of which appear <br> near the base of the cliff; <br> one of them is studded with <br> tarnished crystals of iron py- <br> rites ; another is a smgular <br> mixture of grey limestone <br> and reddish granular gyp- <br> sum The portions of lime- <br> stone contained in this rock <br> do not appear to be frag- <br> ments or pebbles and they <br> are penetrated by plates of <br> selenitic gypsum They may <br> be parts of a bed of limestone <br> broken up and mixed with <br> gypsum when in a soft state <br> or the limestone and gypsum <br> may have been deposited si- <br> multaneously and separated by molecular attraction A rock of this <br> kind is not rare as an accompaniment of gypsum and it may be <br> a Gypsum vein b Open joint <br> c Bedding of the gypsum 35268987 109512 51125 Page 337 Text v 5 http //www biodiversitylibrary org/page/35268987 1849 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 5 1849 Geology Periodicals Smithsonian Libraries bhl page 35268987 dc identifier http //biodiversitylibrary org/page/35268987 smithsonian libraries Information field Flickr posted date ISOdate 2014-02-20 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/12645678164 2015-08-27 10 55 55 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1849 Photos uploaded from Flickr by Fæ using a script | ||||
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