Among the foreign geologists who have been drawn to the Scottish mountains and islands by the interest of their Tertiary volcanic rocks, I have already spoken of Faujas St. Fond. Much more important, however, were the observations made some thirty years later by two German men of science, Von Oyenhausen and Von Dechen. Their careful descriptions of the geology of Skye, Eigg and Arran added new materials to the knowledge already acquired by native geologists.[138] To some of the more interesting parts of their work reference will be made in later pages. [Footnote 138: Karsten's _Archiv_ (1829), vol. i. p. 56.] The numerous trap-dykes of Northumberland, Durham and Northern Yorkshire at an early date attracted the attention of geologists. As far back as 1817, they had been the subject of a memoir by N. J. Winch,[139] who gave an account of their effects on the adjacent rocks. More important were the subsequent papers on the same subject by Sedgwick, who, discussing the lithological characters, probable origin and geological age of the dykes, pointed out that while the Cleveland dyke was undoubtedly younger than a large part of the Jurassic rocks, there was no direct evidence to determine whether dykes farther north were earlier or later than the time of the Magnesian Limestone.[140] Subsequent accounts of the dykes of the same region were given by Buddle,[141] M. Forster,[142] N. Wood,[143] H. T. M. Witham,[144] Tate [145] and others, while in more recent years important additions to our knowledge of these dykes and of their effects have been made by Sir J. Lowthian Bell[146] and Mr. J. J. H. Teall.[147] [Footnote 139: _Trans. Geol. Soc._ vol. iv. (1817), p. 21. See also Tilloch's _Phil. Mag._ vols. xlix. and l.] [Footnote 140: _Cambridge Phil. Trans._ vol. ii. (1827), pp. 21, 139.] [Footnote 141: _Trans. Nat. Hist. Soc. Northumberland_, i. (1831), p. 9.] [Footnote 142: _Op. cit._ i. p. 44.] [Footnote 143: _Op. cit._ i. pp. 305, 306, 308, 309.] [Footnote 144: _Op. cit._ ii. (1838), p. 343.] [Footnote 145: _Trans. Northumberland and Durham_, ii. (1868), p. 30.] [Footnote 146: _Proc. Roy. Soc._ xxiii. (1875), p. 543.] [Footnote 147: _Quart. Journ. Geol. Soc._ xl. (1884), p. 209.]