Dykes formed when molten lava flowed into deep cracks and fissures in pre-existing rocks millennia ago. The lava set in a sheet form within the other rock but frequently dykes are seen on the surface today, after many years of erosion and earth movements, as lines of contrasting rock type. Dykes are composed of many different mineral combinations. When I visited Guernsey in the Channel Islands last year I discovered three types of dyke cutting through the predominantly igneous and metamorphosed rocks. I have already shown some pictures of dolerite, albite dolerite, and lamprophyre dykes, all of which seem to be quite common on the island. However, on an expedition to L’Eree on the north-west coast I spotted a dyke with a very different texture cutting east-west across the other north-south dykes. This proved to be a vogesite dyke.
Vogesite has a very characteristic texture made up of rounded mineral inclusions – ‘large euhedral amphibole phenocrysts’ – set in a fine grained groundmass of plagioclase feldspar, alkali feldspar and quartz (Roach et al. 1991).
REFERENCES
British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.
De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernesiaise, ISBN 0 9518075 2 8, 30-34.
Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, p 22.
Jessica's Nature Blog 
Hi Jessica.
What a pleasant surprise finding your blog. I was a L’Eree headland on Wednesday with a friend of mine who, like me, studied Geology at University back in the 70’s. We are attempting to follow the itineraries set out in the book ‘Outline and Guide to the Geology of Guernsey’ by Roach & others. We were on the lookout for the vogesite dyke and we thought we found it and now confirmed in your photos above. Quite an exciting find and new to both of us. If you are ever back in Guernsey, we would really like to meet up with you to share our knowledge and maybe visit a few more itineraries.
I look forward to hearing from you.
Kind regards
Phil Le Roy
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Hello Phil. Thank you for getting in touch. I am pleased to hear that you found my short piece on the vogesite dyke useful. I enjoyed two visits specifically to search for geological features on Guernsey, following The Rocks and Scenery of Guernsey by de Pomerai and Robinson, which is easier for an amateur like myself to follow, although I also have a copy of the Roach et al publication. It was great fun seeking features out and trying to understand what I was looking at – with greater or lesser degrees of success. It would be delightful to meet up with you and your friend to take advantage of your geological expertise if I ever make it to Guernsey again. So thank you very much for the offer of contacting you.
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