The Isle of Arran lies off the Ayrshire coast in the Firth of Clyde, SW Scotland. James Hutton visited Arran in August 1787 with his companion, John Clerk, and together they made the first geological investigation of the island. Hutton returned to Edinburgh satisfied that he had at last found the critical field evidence he had been searching for in support of his ‘Theory of the Earth’. In June 1797, ten years after Hutton's first survey of Arran, Robert Jameson arrived on the island to investigate its geology and mineralogy. Jameson was still only 22 but had already become one of Hutton's most ardent critics. The previous year he had read two papers to the Royal Medical Society of Edinburgh refuting critical elements of Hutton's ‘Theory’.Based at Kilmichael, near Brodick, on the east coast, Jameson spent most of June, July and August systematically exploring Arran entirely on foot. On returning to Edinburgh, he combined a narrative of his observations on Arran with an earlier trip to his parents' native Shetland Islands, in what became the earliest published account of the geology of Arran; An Outline of the Mineralogy of the Shetland Islands, and of the Island of Arran (1798). Jameson returned to Arran for a second, shorter visit in August 1799 and subsequently published a second book, The Mineralogy of the Scottish Isles … (1800), which presented a revised and updated geological description of Arran.Jameson's handwritten journals for both his 1797 and 1799 tours of Arran survive. In this paper we draw on these first-hand accounts of Jameson's field observations and discuss them alongside his two subsequent books describing Arran's geology. The journals clearly show that Jameson was an acute observer in the field and consistently made his interpretations in accordance with Wernerian principles. Although he had read the published Volumes I and II of Hutton's ‘Theory’, it appears that he had not seen the unpublished Volume III manuscript prior to either tour. Nevertheless, Jameson's exploration of the island closely follows that of Hutton's and they visited many of the same field localities. Here, we use Jameson's Arran journals to retrace his field investigations and discuss them in the light of Hutton's previous work on the island and in relation to Jameson's own Wernerian views. The difference of approach and interpretation between the two in the field on Arran mirrors the wider conflict between Plutonists and Neptunists at this time.