d56 PEOF. J. W. JTTDD Olf THE TEETIAET AND 



Paet I. 

 The Teetiaet Peeidotites a:n-d Allied Eocks. 



§ 1. Relatioi^s oe these Rocks to the othee Ebtjptiye Masses of 

 THE Westeei? Isles of Scotland. 



More than ten years ago I was enabled, by a careful stndy of the 

 Western Isles of Scotland, to demonstrate that the eruptive masses 

 of granite and gabbro of that area constitute the denuded cores or 

 basal wrecks of great volcanoes built up during the earlier Tertiary 

 periods*. As Professor Zirkel had shortly before visited the district 

 for the purpose of collecting specimens of the leading types of rocks 

 in it, and had given a detailed account of their microscopical cha- 

 racters t, I made the petrographical determinations of that excellent 

 observer the basis of my field-work, reserving to a future occasion 

 a full account of the minute structure of the very large series of 

 typical rocks which I collected during my field-work extending over 

 several years, as well as in subsequent visits to the district. In ful- 

 filment of the engagement I then made, I have as yet only been able 

 to deal with the basaltic glasses, in a paper in preparing which I had 

 the cooperation of my friend Mr. Cole t, and I now lay before the 

 Society a second instalment of the task. 



The ultra-basic rocks of the Western Isles of Scotland form sub- 

 ordinate portions of several of the great basic eruptive masses of the 

 Western Isles of Scotland. They are especially developed in the Isle of 

 Eum and in the Shiant Isles. By Macculloch, to whose careful studies 

 we owe so much of our knowledge of the district where they occur, 

 the difference of these rocks from those with which they are associ- 

 ated was clearly recognized, though their true nature was to some 

 extent misunderstood. As we shall hereafter show, the olivine 

 which forms such an important constituent in most of these rocks 

 has undergone a curious modification, by which it is caused to 

 assume a darker colour than is usual in the mineral. In consequence 

 of this change the olivine in many of these rocks appears to the 

 naked eye so similar to the associated augite, that it is not surpris- 

 ing to find Macculloch confounding the two minerals and supposing 

 the rocks to be almost wholly composed of augite. It was in this 

 way that he was led to give them the name of " augite-rock" §. That 

 no subsequent observers have noticed these remarkable rocks is pro- 

 bably accounted for by the fact that the Isle of Rum is seldom visited, 

 while the Shiant Isles are not only almost uninhabited, but are 

 very dif&cult of access. 



The basic eruptive rocks, with which the ultra-basic ones are so 

 intimately associated, fall into the two classes of gabbros and dole- 

 rites. 



* Quart. Joum. Geol. Soc. vol. xxx. (1874) pp. 220-302. 

 t Zeitschr. d. deutsch. geolog. Gesell. vol. xxiii. (1871) p. 1. 

 I Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 444. 



§ A Description of the Western Isles of Scotland, &c., by John Macculloch, 

 M.D. (1819), Tol. i. pp. 436, 485, &c. 



