26 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Among the foreign geologists who have been drawn to the mountains and islands of 

 Scotland by the interest of its rocks, I have already alluded to 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.* To some of the more interesting parts of their work, 

 reference will be made in later parts of this memoir. 



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, 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 the dykes 

 farther north were earlier or later than the time of the Magnesian Limestone. Subsequent 

 accounts of the dykes of the same region were given by Buddle,! M. Forster,| N. 

 Wood,§ H. T. M. Witham,|| Tate,H 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** and Mr J. J. H. Teall.H" 



The geological age of the great series of Tertiary volcanic rocks has only been 

 determined, district by district, and at wide intervals. That some part of the Antrim 

 basalts are younger than the Chalk of that region was clearly shown by Berger, Cony- 

 beare, and Buckland. Portlock, however, in his Report on Londonderry, &c, referred 

 to the occurrence of detached blocks of basalt which he supposed to be immersed in the 

 Chalk near Portrush, and which inclined him to believe that " the basaltic flows com- 

 menced at a remote period of the Cretaceous system." Macculloch showed that the 

 corresponding basaltic plateaux of the Inner Hebrides were certainly younger than the 

 Oolitic rocks of that region. But no nearer approximation to their date had yet been 

 made. In the year 1850 the Duke of Argyll announced the discovery of strata con- 

 taining fossiliferous chalk-flints and dicotyledonous leaves, lying between the bedded 

 basalts of Ardtun Head, in the Isle of Mull. J J In the following year these fossil leaves 

 were described by Edward Forbes, who regarded them as decidedly Tertiary, and most 

 probably Miocene. This was the first palseontological evidence for the determination of 

 the geological age of any portion of the basalt-plateaux, and it indicated that the basalts 

 of the south-west of Mull were of older Tertiary date. Taken also in connection with 

 the occurrence of lignite-beds between the basalts of Antrim, it proved that these 

 volcanic plateaux were not due to submarine eruptions, as the earlier geologists had 



* Karsten's Archiv (1829), vol. i. p. 56. t Trans. Nat. Hist. Soc Northumberland, i. (1831) p. 9. 



t Op. cit., i. p. 44. § Op. cit., i. pp. 305, 306, 308, 309. 



|| Op. cit., ii. (1838) p. 343. 1 Trans. Northumberland and Durham, ii. (1868) p. 30. 



** Proc. Roy. Soc, xxiii. (1875) p. 543. tt Quart. Jour. Geol. Soc, xl. (1884) p. 209. 



XX Hrit. Assoc Report, 1850, sections, p. 70, and Quart. Jour. Geol. Soc, vii. (1851) p. 87. 



