The Reading Room Press

hornfels

In 2018 whilst walking on the north-eastern limit of Orkney Mainland out towards the Brough of Birsay and just about where the M is by the shore on the map below, I came across a slab of rock about 4 feet square and 2 inches thick with the most striking pattern. Too heavy to lift and carry away, my companion on the walk, Patricia Mitchell, took a photograph of it.

Four years later the Senecio Press printed an enlarged section to use as a binding paper for the 2022 publication The Hunting of the Snark.


Intrigued by the unusual and attractive marking (I did take the small piece seen at the bottom of the picture home but its effect seemed to fade) I asked David Dobson, wood engraver, mountaineer and geologist for an explanation:

I think the rock is a slate which might have been close to an igneous intrusion. In that case the high temperatures can produce contact metamorphic minerals and they look like a high-T form of aluminium silicate (Al2SiO5) called andalusite. That normally forms white needles, but at high concentrations it can form radiating mats like those in the image. In that case there should also be some occasional isolated needles. If not andalusite then it is probably the high -P,T form, called sillimanite. You would be safe to call it a hornfels which is a contact-metamorphosed slate.

This explanation seems almost as attractive as the stone itself. Such an esoteric image is well suited the the whimsey of Carroll's Snark.

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