The dark basic rocks which stand out so prominently in the geological structure and scenery of the Inner Hebrides have long attracted notice and have been the subject of frequent description. The writings of Macculloch, Von Oeynhausen and Von Dechen, and J. D. Forbes in the earlier half of this century, and of Prof. Zirkel and Prof. Judd in the later half, have made geologists familiar with the general character and distribution of the Tertiary gabbros. There is one feature, however, in these interesting masses which has hardly received as yet the attention which it deserves. We refer to the frequent bedding and banding which they present. Even from a distance this structure may be distinctly recognized on many of the great declivities. The northern hills of Rum, for instance, can be readily seen to be built up of successive sheets, and on the flanks of the Cuillin Hills in Skye a similar structure may be observed. Macculloch has referred to what he calls the “obscurely bedded disposition” of some of these rocks, and Prof. Judd has stated that “the great masses of gabbro in Rum exhibit that pseudo-stratification so often observed in igneous rocks.” One of the authors of the present paper has drawn particular attention to this structure as evincing that the gabbro masses are not simple eruptive bosses, but are composed of many sills and sheets, the result of successive protrusions of material. He has also dwelt on the fact that not only do these masses exhibit