Mylonites: Lessons from Eriboll

Eriboll has been an important area in understanding the geology and structure of the NW Highlands of Scotland. It came to prominence during the Highland Controversy between Nicol and Murchison in the 1850s. Nicol recognized a major regional, or grand, dislocation (the Moine Thrust Zone) at Eriboll whereas Murchison denied its existence. An important element in the resolution of the controversy was the repeated observation of a distinctive schistose rock that often marked Nicol's dislocation. Lapworth named it a ‘mylonite’ and related its development to mechanical metamorphism resulting from tectonic (milling) movements along the dislocation. Peach, Horne and co-workers, whose contributions to early mylonite studies are often overlooked, described the effects of increasing strain on mylonite development, recorded the prominent lineation within mylonites and identified its kinematic significance via thrust geometries and a first use of shear sense indicators. In this contribution those elements of the Highland Controversy that led to the identification of mylonites will be reviewed, concentrating on the contributions of Lapworth and of Peach, Horne and co-workers. It will include an examination and discussion of the mylonites from Lapworth's locality and of the mylonitized Moine schists at Eriboll. It will conclude with comments on mylonite classification and nomenclature