56 PEOP. J. W. JIJDD ON TEETIAET 



dering is thus seen to extend, from north to south, over a distance 

 of 700 miles, or ten degrees of latitude. 



III. Geological Age. 



There is one question which lies at the very foundation of my 

 arguments on this subject, that, namely, of the true geological age 

 of the rocks, and this question must therefore be dealt with at the 

 outset. Is it established beyond all possible doubt that the rocks 

 were really erupted in Tertiary times? As there still appear to 

 prevail unfortunate misconceptions on this point, and some con- 

 tinental geologists have given extensive currency to the opposite view, 

 it may be necessary to recapitulate, as briefly as possible, the evi- 

 dence upon the subject — evidence which I hold to be absolutely 

 conclusive. 



Dr. MaccuUoch, in 1819, clearly showed that the basalts of the 

 Western Isles of Scotland are most intimately associated with the 

 rocks we now designate as " gabbros " ; he characterized the whole of 

 these " trap rocks " as an overlying formation, which was proved to 

 be of later date than all the Secondary strata of the district, not 

 only by the fact that it rests on these latter, but by its sending off 

 numerous apophyses which penetrate and alter them. He went 

 further than this and insisted, as the result of his studies, that the 

 period of the eruption of the whole of the igneous masses was sepa- 

 rated from that of the deposition of the Secondary strata by a great 

 interval*. Murchison subsequently showed that the Secondary 

 strata referred to by Macculloch include various members of the 

 Lias and Oolite ; he even thought that he had detected representa- 

 tives of the Wealden, but these have since been shown to be of 

 Oxfordian agef. 



In 1851, Professor Edward Porbes announced that the plant- 

 remains of Ardtun which had been found by the Duke of Argyll 

 in deposits interbedded with the basalts of Mull, are of Tertiary 

 aget. Shortly afterwards, he was able to correct Murchison's 

 identification of certain beds in Trotternish, in Skye, with the 

 Wealden § ; but in doing so, he suggested the possible contem- 

 poraneity of the basaltic rocks of that area with the Jurassic 

 deposits so intimately associated with them, illustrating these views 

 by a diagrammatic section. It must be remembered, however, 

 that Edward Eorbes, as he himself tells us, had no opportunity of 

 studying the relations of these rocks beyond that afforded by a short 

 yachting cruise, and that he expressly cautions his readers against 

 accepting his conclusion as being anything more than a suggestion. 

 He writes as follows : — 



" Another view may be taken, however, of the origin of the 

 basaltic sheet intervening between the Upper and Middle Oolites in 

 Skye, one which would seriously affect the preceding estimate of its 



* Western Isles of Scotland (1819), vol. ii. p. 67, &e. 

 t Trans. Geol. Soc. 2nd ser. vol. ii. p. 292, and ibid. p. 353. 

 X Quart. Journ. GeoL Soc. vol. vii. (1851), p. 103. 

 § Ibid. pp. 101-117. 



