ABSTRACT New zircon U-Pb dates from the Mount Fitton, Mount Sedgwick, Mount Schaeffer, Old Crow, and Dave Lord plutons indicate that granitoids of the Old Crow plutonic suite in northern Yukon were emplaced in the North Slope subterrane of the Arctic Alaska composite terrane between 375 ± 2 Ma and 368 ± 3 Ma. Whole-rock major and trace element and Nd-Sr isotope geochemistry, combined with zircon trace element and Hf isotope geochemistry, indicate magma genesis involved significant contribution from older continental crust. Samples from the five plutons yield whole-rock εNd (t) values from -3.9 to -11.6 and 87 Sr/ 86 Sr (i) ratios of 0.7085–0.7444 and 0.8055. Zircon εHf (t) values range from -6.2 to -13.3. These North Slope subterrane granitoids are generally younger and isotopically more evolved than felsic rocks in the Coldfoot and Hammond subterranes of the southern Brooks Range (Arctic Alaska terrane), but in part are coeval with felsic rocks on the Seward and Chukotka peninsulas. The North Slope granitoids are also coeval and geochemically similar to arc magmatism in the Yukon-Tanana terrane in Yukon and on Axel Heiberg and northern Ellesmere islands, Nunavut. The Old Crow plutonic suite is interpreted as part of a Late Devonian arc system developed along the Arctic and Cordilleran margins. Late Devonian plutons were most likely emplaced after initial translation of the North Slope subterrane along the northern Laurentian margin. The plutons lie within or north of the Porcupine shear zone and thus do not limit post-Late Devonian displacement on the boundary between the North Slope subterrane and northwestern Laurentia.

Abstract For some forty years after Adam Sedgwick took to the field in the Lakes in 1822, he was virtually the only major geologist to make the region one of his main objects of study. We have seen how he made considerable efforts to sort out the stratigraphy of the southern Lakes, relating his work there to his investigations in Wales, but to a large extent he left the central volcanic region alone; likewise the Skiddaw Slates - apart from claiming them for his Cambrian System on the basis of their apparently meagre fossil contents, as collected by Ruthven. The man who took up from where Sedgwick left off was Robert Harkness (1816–1878) (see Fig. 3.1 ). 1 His family came from Ormskirk, near Liverpool, but he attended school in Dumfries in SW Scotland, his father’s home town. From there he went to Edinburgh University, where he studied under the geologists Robert Jameson and James David Forbes, and the chemist Thomas Charles Hope. In his early twenties Harkness was making geological investigations in Lancashire, particularly among the coalfields and the rocks of the New Red Sandstone. In 1848, the family moved to Dumfries, from which centre Harkness, being of independent means, began geological investigations among the rocks of the Southern Uplands, including the region near Moffat, which, rich in graptolites, later became a classic site through the work of Charles Lapworth (1842–1920) ( Hamilton 2001 ). Thus early in his career, Harkness became familiar with these Lower Palaeozoic fossils. In fact, his