The Isle of Rum contains one of the most interesting assemblages of plutonic rocks of Tertiary age in the British Isles. The pioneer work of Macculloch and the later exploratory efforts of Judd and Geikie were followed by an admirable account of the rocks by Harker, given with his usual lucidity and precision. In 1938 I spent a fortnight on this island, and this paper—the outcome of my visit—aims merely at supplementing Harker's petrographical description by certain quantitative data in respect of the ultrabasic and basic rocks found there and of recording certain observations relating to the igneous tectonics.

The earliest phase of igneous activity on Rum was the eruption of doleritic and mugearitic lavas. As shown by Bailey the eruptive phase of igneous activity was followed or accompanied by the formation of a ring-fault and two subordinate vents iufilted with explosive breccia and intruded by felsite. The space inside the ring-fault was then intruded by a banded peridotite-allivalite complex and later by eucrite in the form of a semicircular ring-dike and associated sheets. Finally, a granophyre boss was intruded along the western side of the ultrabasic-basic complex.

Palmer's investigation of the chemical and mineralogical aspects of soil-forming processes was restricted to comparative chemical analyses of the weathered shells and the essentially unaltered cores of certain spheroidal basic boulders from Wahiawa in the Hawaiian Islands.

The present writer describes an experimental approach to the general problem of rock-weathering on lines suggested by Dr. A. Brammall:

1. (a) Electrodialysis of eight typical rock-forming mineral silicates, each of which was dialysed separately after having been ground to an impalpable powder and analysed in detail.

2. (b) Assessment of the effect of dialysis by comparing the composition of the unaltered mineral with that of the slime left in the mineral-chamber when dialysis was virtually at an end.

3. (c) X-ray and kathode-ray tests on three of these minerals and the corresponding slimes, to ascertain whether colloidal end-products associated with, or coating, particles of unaltered mineral had integrated to form one or other of the specific ‘crystalline colloids’ identifiable in soils and clays.