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Arthur's Seat: Hutton's Section

A classic site showing the contact between a transgressive teschenite sill and the underlying cementstone strata (thinly bedded shales). At the base, the sediments dip eastwards at c. 20°. The sediments close to the contact have been baked, and are consequently harder. Along the line of contact is a thin film of greenish rock. This represents natural basaltic glass (tachylyte) resulting from rapid chilling of the intruded magma. This layer is now de-vitrified and decomposed.

Immediately above the contact the sill-rock is very fine-grained and displays 'knurled jointing' as a result of relatively rapid cooling; crystal size and vesicle size increased notably upwards. The sill is generally conformable with the sediments, but at this section undergoes two small transgressions. At the more southerly and more conspicuous of these, a block of cementstone has been prised upward by intrusion of a teschenite wedge.

The sill at this point shows 1.5 m of lower marginal facies (closely-spaced joints), overlain by 6.0 m of columnarly-jointed sill rock, with a further 1.2 m of closely-jointed upper marginal facies a the top, though the overlying cementstone are unfortunately absent. Tabular cementstone xenoliths occur in the sill SE of the section.

The section has historical associations with James Hutton (1726-1797), the founder of modern geology and first true protagonist of uniformitarianism. Hutton argued for an igneous origin for sill rocks and lavas; the countering opinion, that of the Neptunists or Wernerians, was that all rocks had precipitated out at the base of a primeval 'universal ocean'. The evidence for baking and chilling evident at the contact in this section provided Hutton with some of his ammunition for falsifying the Wernerian paradigm.

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What distinguishes the sill rock here from a lava flow? (Answers)
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