OLDER PERIDOTITES OF SCOTLAND. 357 



The gabbros, as Zirkel so well showed *, belong for the most part 

 to the class of the olivine-gabbros, the olivine in these rocks, how- 

 ever, being frequently so curiously obscured, as to be easily over- 

 looked. 



By other writers these rocks have been called hypersthene-rock f, 

 hypers thenite or hyperite +, hyperitic diabase §, labrador-syenite ||, 

 and norite %. 



The main cause of the conflict of opinion concerning the nature 

 of this rock and the name by which it should be called was the 

 difficulty found in determining its conspicuous pyroxeuic constituent. 

 As we shall show hereafter, Zirkel was undoubtedly right in referring 

 this pyroxene to diallage, the foliated form of augite **. At the same 

 time it must be borne in mind that the various forms of enstatite, and 

 among them true hypersthene, are also frequently present, and some- 

 times in such proportions as to entitle the rock to be called an 

 " olivine- enstatite-gabbro." 



It is a surprising circumstance that some authors have regarded 

 the rock in question as being of metamorphic origin and of Archaean 

 age. The only possible ground for such a conclusion which I can 

 conceive of is that the rocks present certain parallel division-planes, 

 and are built up of the same minerals as some supposed Lauren- 

 tian rocks. As for the parallel division-planes they are certainly 

 nothing but the great concentric joints which are so often found tra- 

 versing both gabbros and granites ; while, as I have elsewhere shown,. 

 the connexion of these rocks with the Post-Tertiary basalts is un- 

 mistakable. 



As, however, the supposed metamorphic origin and Archaean age 

 of the gabbros of the Cuchullin Hills of Skye has not only been 

 taken for granted, but has been made the basis of curious theo- 

 retical speculations, it may be well to trace the idea to its original 

 source. 



The first who broached these notions appears to have been Dr. 

 A. Geikie ft, before whom, so far as I am aware, no one had ques- 

 tioned the reference made by Macculloch of these rocks to the 



* Zeitscbr. d. deutsch. geol. Gesell. xxiii. (1871) p. 58. In 1878, Prof, 

 von Lasaulx gave an interesting account of the microscopical characters of the 

 similar rocks of the Oarlingford Mountains, which form, as I have shown, a 

 portion of the same great Tertiary series of outbursts. (Tschermak, Min. u. Petr. 

 Mitth. 1878, p. 426.) His results are in close agreement with those of Prof. 

 Zirkel. 



t Macculloch, ' Western Isles,' voL i. p. 385 ; Geikie, ' Scenery and Geology 

 of Scotland ' (1865), p. 210. 



t Heddle, Trans. Eoy. Soc. Edin. vol. xxviii. (1879) p.. 478. 



§ Heddle, loc. cit. vol. xxviii. (1879) p. 252. 



II Haughton, Dubl. Quart. Journ. Sc. vol. v. (1865) p. 94. 



^ Sterry Hunt, Chem. & Geol. Essays, 2nd ed. (1879) p. 281. 

 ** Dr. Heddle has arrived at the startling conclusion that not onfy is diallage 

 never found in . these rocks, but that in Scotland this mineral occurs only in 

 metamorphic rocks! (See Trans. Eoy. Soc. Edinb. vol. xxviii. (1879), p. 477 

 footnote.) 



tt Trans. Roy. Soc. Edinbw vol. xxi. (1861) p. 633; also ' Scsnery and Geology 

 of Scotland' (1865), p. 210, footnote, and Murchison and Geikie s Geological 

 Map of Scotland, 1st edition, 1865. 



Q.J. G.8. No. 163. 2 c 



