408 



Scientific Proceedings, Royal Dublin Society. 



felspathic constituent was a Labradorite. Some few well-developed, 

 lath-shaped sections, parallel to the M-face, give an extinction of 

 - 30° : this denotes the presence of a felspar allied to Abi An 3 . 

 Such sections show the albitic lamellation. The mass of the 

 felspar is, however, particularly striking owing to the want of 

 this striated appearance, to its occurrence in a granular condition, 

 and to its association with quartz and epidote. This combination 

 of minerals forms, under crossed Nicols, the so-called quartz- 

 felspar- (epidote) -mosaic : it bears every aspect of being secondary, 

 and recalls the metamorphism of labradorite (Abi An 2 according 

 to Schilling l ) into albite with bye-products, as has been described 

 by Lossen in the case of the Hartz diabases 2 . The similarity 

 being so strong, we may, therefore, assume that a like alteration 

 has been effected in the felspar of our Irish Epi-diorites. 



The albitic character of the secondary felspar has been inferred 

 from the analyses of the vein-felspars so frequently observed in 

 rocks of this class. Giimbel 3 supplies, for instance, an analysis 

 of a vein-" albite " from the Fichtelgebirge, whilst Teall 4 gives 

 the composition of the vein-felspar in the Scourie Dyke in 

 Sutherlandshire as that of andesine. From his analysis I have 

 calculated the admixture of orthoclase, albite, and anorthite, to be 

 as follows : — 



1 Griinsteine des Harzes : Gottingen, 1869. 



2 Studien an metamorphischen Eruptiv-und Sedimentgest. Jahrbuch d. k. preuss. 

 geol. Landesanstalt fur 1883 : Berlin, pp. 619-640 ; also Erlauterungen zur geol. 

 Specialkarte von Preussen. Blatt "Wippra, 1883, pp. 50 and S3. 



3 Die palaolithischen Eruptivgesteine des Fichtelgebirges, 1874, p. 14. 



4 British Petrography, p. 155. 



