Dr. J. 8. Hi/ land — Epi-diorites in Ireland. 207 



control of powerful mechanical stresses." l The original felspathic con- 

 stituent 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 Ab, An 3 . Such 

 sections show the albitic lamellation. The mass of the felspar is, 

 however, particularly striking owing to the want of this striation, 

 to its granular condition, and to its occurrence in conjunction 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 (Ab t An 2 according to Schilling 2 ) into albite with 

 bye-products, as has been described by Lossen in the case of the 

 Hartz diabases. 3 The similarity being so strong, we may therefore 

 assume that a like altei-ation 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 4 supplies, for instance, an analysis of 

 a vein-" albite " from the Fichtelgebirge, whilst Teall 5 gives the 

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

 shire as that of andesine. From his analysis I have calculated the 

 admixture of orthoclase, albite, and anorthite to be as follows : — 



2-4 1 



This would represent a felspar of the constitution An^ Ab,. 4 , i.e. 

 a felspar intermediate between andesine and oligoclase. 



It would manifestly be more accurate to analyse the felspar which 

 occurs in the ground-mass of the rock itself; but no attempt has 

 apparently been made in this direction owing to the difficulties 

 the isolation presents. In order to do this, the rock from Convoy 

 was taken, and, after being powdered, treated with weak acid in 

 order to remove the small quantity of calcite present. The secondary 

 felspar was then separated by means of the Sonstadt-solution : its 

 specific gravity was found to be 2-645. The analysis gave the follow- 

 ing result : 



1 Judd, "On the Processes by which a Plagioclase Felspar is converted into 

 a Scapolite," Min. Mag. viii. No. 39, p. 186 (13). 



2 " Griinsteine des Harzes," Gbttingen, 1869. 



3 " Studien an metamorphischen Eruptiv- und Sedimentgest," Jahrbuch d. k. 

 preuss. geol. Landesaustalt fur 1883, Berlin, pp. 619-640. Also, Erlauterungen 

 zur geol. Specialkarte von Preussen, Blatt Wippra, 1883, pp. 50 and 83. 



4 " Die palaolitbischen EruptivgesteinedesFichtelgebirges," Miinchen, 1874, p. 11. 



5 British Petrography, p. 155. 



