390 SIE A. GETKIE ON THE TEETIAKT [May 1896, 



rocks in place, and, in spite of their apparent testimony to the 

 posteriority of the acid intrusions, he was inclined to believe that 

 the veins were not real veins, but that the ' trap' and ' syenite ' had a 

 common origin and would be found to pass into each other, as he 

 thought also occurred in Mull and Rum. In recent years 

 Mr. Alexander Ross has visited St. Xilda and published an excellent 

 account of its geology. 1 He collected specimens illustrating the 

 varieties of gabbro, dolerite, and basalt, and showing the intrusion 

 of the acid into the basic rocks. He was disposed to believe the 

 'granite' to be of younger date than the gabbros, but left the 

 question open for further consideration. 



The acid rock which forms the eastern side of this island, variously 

 termed ' syenite ' and ' granite,' weathers in thick bed-like sheets, 

 divided by transverse joints into large quadrangular blocks, like 

 many granites. On closer inspection it is found to resemble still 

 more precisely the acid rocks of the Inner Hebrides. It possesses 

 the same drusy micropegmatitic structure as the granophyres of Skye, 

 Rum, and Mull. The ferro-magnesian constituents are present in 

 small quantity, hence the pale hue of the stone. The quartz and 

 felspar project in well-terminated crystals into the drusy cavities, 

 which are sometimes further adorned with delicate tufts of clear, 

 crystallized epidote. In many respects the rock resembles the 

 young granites of Arran and the Mourne Mountains. 



Mr. Harker's notes on the microscopic structure of this granophyre 

 are as follows: — 'The prevailing felspar is orthoclase, often very 

 turbid from secondary products. Even what appear to be distinct 

 crystals are sometimes seen in the slices to be invaded in the margin 

 by quartz in rough micrographic intergrowths, and much of the finer 

 intergrowth occurs as a fringe to the crystals. In this case the 

 felspar of the micropegmatite can often be verified to be in crystalline 

 continuity with the crystal which has served as a nucleus [6624]. 

 Quartz occurs in distinct crystals and grains as well as in the micro- 

 pegmatite. There is a more granitoid variety of the rock, in which 

 only a very rude approach to micrographic intergrowths is seen 

 [6623]. In both varieties there is but little trace of any ferro- 

 magnesian mineral ; the more typical granophyre has what seems 

 to be destroyed augite, while the granitoid rock contains a little 

 deep-brown biotite. Scattered crystal-grains of magnetite occur 

 in both.' 



Narrow ribbon-like veins of a finer material, sometimes only an inch 

 in breadth, traverse the ordinary granophyre. Similar veins run 



1 Brit. Assoc. Rep. 1885 (Aberdeen meeting), p. 1040 ; and a much fuller paper 

 in the Proc. Inverness Field Club, vol. iii. (1884) p. 72. In this latter paper 

 a letter from Prof. Judd is quoted, in which he states that the rock supposed 

 to be granite 'is seen under the microscope to be a quite different rock — a 

 quartz-diorite,' p. 78. Some of the specimens from St. Kilda collected by 

 Mr. Ross were exhibited at the meeting of this Society on January 25th, 1893. 

 With regard to these, Prof. Judd, in the course of the discussion on his paper 

 on ' Inclusions of Tertiary Granite in the Gabbro of the Cuillin Hills,' 

 remarked : — ' They show a dark rock traversed by veins of a light one, but the 

 dark rock is not a gabbro, and the light rock is not a granite/ Quart. Journ. 

 Geol. Soc. vol. xlix. (1893) p. 198. ■••■■>. 



