it and without fossils. If you break the stone open, though, as Mike Henderson showed me, you can find the ghosts of the life once preserved here. Heat and pressure has transformed everything that was once in them and a faint, coppery, gilded sheen coats the inner surfaces of the rock. These are what Mike Henderson called ‘the pyritised remains’, the only evidence of the fossils which the rock once held, reduced in the furnace to this undiagnosable inkling, a breath of a suggestion, like the ash of an abandoned hearth. But the mud stones also exacted their price from the magma. Where the hot, molten intrusion came into contact with the country rock it was suddenly chilled. The semi-crystalline mush of the liquid, a kind of hot granita, was shocked into solidity by the old, cold stones. In suddenly becoming solid, the minerals in the magma are small, almost instantly formed and so the rock is as smooth and as fine-grained as cheese. That analogy is curiously exact: where you have broken the rock open, the fracture marks are exactly like those you find on the open face of a piece of broken Parmesan. ‘Chill’ or ‘chilled margin’, as this frozen magma is called, sits all alongside the hornfels on the beach, the twin products of intense heat meeting ancient cold, fifty-eight and a half million years ago.
Many things are still not clear about the making of these islands, nor is the precise order of events certain. Broadly though, it seems that several blades or ‘sills’ of magma were intruded here over a period of perhaps a century or two. One fairly narrow one, having cooled quite quickly, can be seen in the Galtas. Another makes the rib of rock through which the natural arch passes at the north-east corner of Garbh Eilean. A third forms Eilean Mhuire and the fourth, by far the largest, 537 ft. thick, created the vast bulk, about four hundred acres, of Garbh Eilean and Eilean an Tighe. No one has yet found how these sills are related to each other, nor how they are connected to the huge feeder pipe along which the magma arrived from the south, but it is obvious that some of the old Jurassic rocks, the mudstones and shales, were caught up in the process. Most of the top of Eilean Mhuire consists of a huge raft of those older rocks buoyed up like a baulk of wood on the flood of magma which poured in, mostly below but some also above it. Another slab of old rock crosses the north-east corner of Garbh Eilean and has been eroded in the bay that stretches out below the puffin slopes there. Those soft and crumbly rocks, full of the nutritious minerals which accumulated on the Jurassic sea-bed, make far better soils than the hard, solidified magma of the sills. These Jurassic rocks, richer than almost anything in Pairc or Harris, and comparable in Lewis only to equivalent rocks around Stornoway, have been the basis for most of the farming on the Shiants. Without the wealth of Eilean Mhuire and the beautiful meadows at the Bagh on Garbh Eilean, it is difficult to think that life on the Shiants would ever have been possible.
For the professional geologists, this story is only the introduction to the book. They seek to penetrate much further into the arcana of chemical detail, above all into the diagnostic mineralogy of the different sorts of rocks to be found on the islands.
Previously it had been thought that the different minerals that can be seen in different places were the result of large crystals settling towards the bottom of the sill as the magma cooled. That was the old orthodoxy developed in the 1930s but Fergus Gibb and Mike Henderson have over the last four decades pushed most of that aside and discovered a quite different process.
As they have revealed, the enormous main sill of Garbh Eilean and Eilean an Tighe is itself made up of at least four different pulses of magma. These were not violent events but of great scale and immense power, slowly applied; a slow squeezing apart of an abscess the size of fifty city blocks. The Shiants laid on Manhattan would stretch from Wall Street to Times Square. These successive pulses are not, as you might think, laid one on top of another, in the way you would assemble a sandwich: mustard on top of the ham on top of the cheese on top of the butter on top of the bread. Not at all: each new one inserted itself, in a hot, licking tongue of new magma, within the body of those that had come before. They could only have done that if the preceding magma was still quite soft. In other words, here in this main sill, they must have followed, one from the other, quite quickly. The picture is of a gradually fattening sandwich, made in something like a piece of pitta bread. You begin with the bread alone, and one by one, butter, jam and peanut butter are squeezed inside it. The sequence you end up with, then, is: bread, butter, jam, peanut butter, jam, butter, bread.
In the Shiants, although the sandwich is 537 ft. thick, it is almost precisely like that, made up of four interleaved layers. The first, of a rock called teschenite, was 6 ft. thick (the top and bottom of this layer are now 531 ft. apart), the second (picrite), 78 ft. thick, the third (crinanite-picrodolerite), 440 ft. and the fourth (granular olivine picrodolerite), just 13 ft.
Beyond that, I fear, it becomes difficult for a layman to follow. I could tell you about the mineralogy of these islands, with the intriguing differences between picrite (which cooled slowly, so has big crystals and looks wormy, with an eroded surface like the mottled, liver-spotted skin of a toad or like penne in a pesto sauce) and picrodolerite (quicker cooling, a fine, granular porridge), or about the beauty in freshly broken rock of the tiny grape crystals of the olivine and the big black glitter of the pyroxenes, about the evolution of the magma over the years of its emplacement, so that the last spurts of it produce a strange, white, open-structured, vast-crystalled rock called syenite on Eilean Mhuire – but I won’t. This is not the place.
After the magma from the vast chambers below had finally exhausted itself, the Shiants, still deep underground, began to cool. From both above and below, the chill started to reach inwards into the heart of the semi-liquid mush, like the hemlock in the limbs of Socrates. It might have taken a century or so for the rock to become solid. As it cooled, it shrank, and that is the origin of the columns of which the Shiants are made. Any large body of shrinking material, contracting over its entire width, pulls apart from itself internally. Shrinkage cracks develop in the body of the rock identical to the network of polygons that develop on the floor of a drying lake. The columns are nothing more than a network of cracks extended into a third dimension.
If this cooling had been conducted in laboratory conditions, where the magma sheets were of equal thickness throughout and both upper and lower surfaces of the intrusions were level, then a structure of complete regularity would have emerged. All the columns would have been straight, the same size, and parallel. But this is not a laboratory. Some of the intrusions were clearly thin (in particular the one that formed the Galtas) and cooled more quickly. This has meant that the columns are themselves much thinner there. More intriguingly and beautifully, it is clear that the opening into which the magma squeezed was uneven. The columns would have formed by growing perpendicular to the cooling surfaces and here the unevenness of those surfaces has created columns that curve, twist and bend, are waved like the hair of art deco statuettes, fixed in the elegance of a geological perm, Jean Harlow turned to stone, Madonna having glimpsed the Gorgon.
Others, such as the upper sections of the north cliffs on Garbh Eilean, cooled so quickly that whole slaggy masses of rock became solid before columns could develop. Below them, deeply buried in the huge Garbh Eilean sill, the magma cooled very slowly indeed and here the Shiant rock-forms attain the great magnificence of the giant columns. These were the forms over which John Macculloch, the early geologist, enthused in 1819:
The lover of picturesque beauty will here, as in many other parts of the Western islands, be gratified with a display of maritime scenery combining the regularity of Staffa with the grander features of the coast of Sky. Towards the north it exhibits one continuous perpendicular face of naked rock. This face is columnar throughout, and forms a magnificent scene for the pencil; spreading in a gentle curve for a space of 1000 yards or more, and impending in one broad mass of shadow over the dark sea that washes its base. In simplicity and grandeur it exceeds Staffa almost as much as it does in magnitude; offering to the tourist an object as worthy of his pursuit as that celebrated island, and of no very difficult access from the northern extremity of Sky.
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