In the year 1879, during a traverse of some portions of the volcanic region of Wyoming, Montana and Utah, I was vividly impressed by the identity of structure between the basaltic plateaux of these territories and the youngest volcanic areas of Britain. It then appeared to me that some of the puzzling features in the Tertiary volcanic series of the Inner Hebrides might be explained by the structures so admirably displayed in these lava-fields of the Far West.[156] Riding over the great basalt-plains of the Snake River and looking at the sections cut by the river through the thick series of horizontal basalt-beds, I appreciated for the first time the significance of Baron von Richthofen's views regarding "massive" or "fissure" eruptions, as contradistinguished from those of great central cones of the type of Etna or Vesuvius, and I gathered so many suggestions from my examination of these American regions that I renewed with increased interest the investigation of the Tertiary volcanic tracts of Britain. At last, after another interval of nine years, during which my weeks of leisure were given to the task, I was able to complete a discussion of the whole history of Tertiary volcanic action in this country, which was communicated to the Royal Society of Edinburgh in the early summer of 1888.[157] Since that time I have continued the research, and have from time to time communicated my results to the Geological Society. These various memoirs are combined with hitherto unpublished details in the following account of the British Tertiary Volcanic Rocks. [Footnote 156: _Geological Essays at Home and Abroad_ (1882), pp. 271, 274; _Nature_, November 1880.] [Footnote 157: _Trans. Roy. Soc. Edin._ vol. xxxv. part ii. (1888), pp. 23-184.] Professor Judd has also prosecuted the investigation of the petrography of the rocks, and has published his observations in the _Quarterly Journal of the Geological Society_.[158] To these papers by him more detailed reference will be made in later Chapters. [Footnote 158: _Quart. Journ. Geol. Soc._ vols. xlv. (1889), xlvi. (1890), xlix. (1893). In the first of these volumes Professor Judd offered a detailed criticism of my views as to the order of succession and history of the volcanic rocks of the Inner Hebrides. Subsequent investigation having entirely confirmed my main conclusions, it is not necessary to enter here upon matters of controversy. Reference, however, will be made in subsequent Chapters to some of the points in dispute.]