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| Keywords: The Quarterly journal of the Geological Society of London (13937286652).jpg 470 ME S S BfCIOIAX OX THE <br> Considering the very great development of the Toarcian limestone <br> in the Cotteswolds it may be advantageous to give a comparison of <br> the strata of that district with a section of the strata in Dorset fig 2 <br> facing p 456 This correlation is entirely founded on the position of <br> the different species of Ammonites ; and for further information on <br> this matter the reader is referred to my Ponograph on Inferior- <br> Oolite Ammonites Palaeontographical Society p 91 <br> The horizontal diagram fig 3 p 468 is intended to represent the <br> Inferior-Oolite strata north and south of the Mendips with their <br> geographical extension according to the correlation just given on a <br> line from Leckhampton Hill to Burton Bradstock Apparently the <br> Mendip range must have acted as a barrier between the areas on <br> each side of it but it is more curious to find that a change in litho- <br> logy in the Sodbury district a change which I have demonstrated in <br> the case of the sands should coincide with an outcrop of Carboniferous <br> Limestone This is part of a patch which extends to Charfield and <br> reappears across the Severn and no doubt formed at that period a <br> subsidiary range of hills joining the Mendip axis at right angles <br> However the character which this figure brings into greatest <br> prominence is the remarkable persistence of a hiatus or lacuna <br> due to the absence of certain strata in the middle of what is called <br> Inferior Oolite This is the same hiatus which Eugene Deslongs- <br> champs finds in Xormandy and which he proposes shall mark the <br> uppermost limit of his Infra-Oolitic marls ; this is the same hiatus <br> which Dr Yacek says occurs over the continent generally and <br> which he proposes as the uppermost limit of the Lias ; this is the <br> hiatus which I following d'Orbigny propose to take as the dividing <br> line between the Toarcien and the Bajocien ; this is the hiatus <br> which is accompanied palaeontologically by the sudden exit of nearly <br> all the Hildoceratidae or ' true Falciferi <br> One last subject remains to be dealt with namely the matter of <br> mapping In connexion herewith the proposed classification would <br> introduce some advantages and would certainly get rid of such <br> anomalies as the same zone being mapped as Mid ford Sands <br> in one county and as Upper Lias Clay in another or again as <br> Midford Sands at one place and Inferior-Oolite Limestone some- <br> where else It would probably be best for this purpose to divide <br> the Toarcian into Upper and Lower although there is between <br> them lithologically no marked break at any point and palaeonto- <br> logically only a small one This palaeontological break occurs at <br> the end of the Opalinum-zone with the extinction of the genus <br> Grammoceras ” only one species of which I believe survives into the <br> Murchisonce-zone Therefore the Falciferum- Commune- Jnrense- <br> and Opalinum-zon es form the Lower Toarcian They would <br> require the following changes among others namely the abolition of <br> the dividing-line now drawn between Upper Lias and Midford <br> Throughout a large part of Dorset and Somerset the Upper Lias Clay is <br> not mapped although to a great extent it overlies what is marked as 2 <br> Middle Lias For instance at South Pethertom it is exposed to a depth of <br> nearly seven feet on the top of the Marlstone ; but nothing is said about it on <br> the map 36940297 113696 51125 Page 468 Text 45 http //www biodiversitylibrary org/page/36940297 1889 Geological Society of London NameFound Grammoceras NameConfirmed Grammoceras EOLID 4786067 NameBankID 4181408 NameFound Hildoceratidae NameConfirmed Hildoceratidae EOLID 4785987 NameBankID 5051689 Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 45 1889 Geology Periodicals Smithsonian Libraries bhl page 36940297 dc identifier http //biodiversitylibrary org/page/36940297 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 N02/13937286652 2015-08-26 05 52 55 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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