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| Keywords: The Quarterly journal of the Geological Society of London (13889246257).jpg 540 <br> H HICKS ON THE PRE-CAMBEIAlf <br> Evidence bearing on another important question already touched <br> upon is furnished in the face of the cliffs here as not only has the <br> Dimetian been penetrated in all directions by diabase dykes but also <br> by dykes of porphyritic quartz-felsites like those described in Porth- <br> clais farmyard in the fields near Porth-melyn and as cutting across <br> the Pebidian rocks near Nun's Chapel famishing additional proof <br> against the view that the porphyritic felsites are in any way to be <br> associated with the Dimetian rocks as peripheral masses A slide <br> from a specimen taken from one of these dykes in the Dimetian at <br> Ogof-Uesugn is described in Note 7 I have not found any of <br> these porphyritic quartz-felsites cutting actually into the Cambrian <br> rocks but diabase dykes differing somewhat however in appearance <br> from the majority of those found in the Pre- Cambrian rocks occur <br> plentifully in them The alteration produced by the diabase dykes <br> in the beds they cut across and the welding which takes place <br> may be studied with advantage in the cliffs along the coast between <br> Ogof-Uesugn and Porth-clais One of these dykes is about 40 feet <br> across and the alteration produced by it in the rocks on either <br> side is most marked <br> 5 PortTi-lisky and Ramsey -Sound Area <br> On the west side of the Dimetian the Pebidian beds lie at a hio-h <br> angle with a dip towards the jST W Immediately at their point of <br> contact with the Dimetian there are the usual indications of a <br> fault and doubtless a considerable thickness of the lowest beds has <br> been dropped by it That these are some of the lower beds of the <br> Pebidian not to be confounded with those under the conglomerate <br> in Ramsey Sound with which they are correlated by Dr Geikie <br> is perfectly clear from the stratigraphical evidence here and at many <br> other places in the area further north ; the microscopical evidence <br> also is most conclusive on this point The sketch fig 13 shows <br> Pig 13 ” Blcetch of Cliffs in East Harbour Porth-lislcy showing <br> contact of Pehidlan ivith Dimetian and line of Fault between <br> F Fault <br> - - Piabase dyke 36941373 113697 51125 Page 540 Text 40 http //www biodiversitylibrary org/page/36941373 1884 Geological Society of London Biodiversity Heritage Library The Quarterly journal of the Geological Society of London v 40 1884 Geology Periodicals Smithsonian Libraries bhl page 36941373 dc identifier http //biodiversitylibrary org/page/36941373 smithsonian libraries Information field Flickr posted date ISOdate 2014-04-30 Check categories 2015 August 26 CC-BY-2 0 BioDivLibrary https //flickr com/photos/61021753 N02/13889246257 2015-08-26 05 53 08 cc-by-2 0 PD-old-70-1923 The Quarterly journal of the Geological Society of London 1884 Photos uploaded from Flickr by Fæ using a script | ||||
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