H. G. Sargent — The Penmaenmawr Intrusions. 21 



attention to them there, and described their structure thirty years 

 ago. They are called by the quarrymen 'spar'. In other parts of 

 the mass the green veins and patches are common, and the light- 

 coloured 'spar' is nearly absent. Schaub * classifies and describes 

 these veins and patches under five heads, according to their colour 

 and grain, but, as there is no essential difference in structure, and the 

 different varieties can be observed in the field to merge into each other 

 in the course of a single vein, this classification seems unnecessary. 



All these veins and segregation patches afford beautiful examples 

 of micropegmatite, which forms the greater part of the vein-material. 

 Thin sections show the presence also of orthoclase and plagioclase 

 crystals (the latter in subordinate quantity), augite, biotite, apatite, 

 ilmenite, and secondary epidote. There is also frequently present 

 a colourless mineral, showing radiating structure and with high 

 indices of refraction and double refraction, which Schaub determines 

 as prehnite, and considers to be an alteration-product of plagioclase. 

 On the authority of Brauns he explains its presence as follows: — 3 



Plagioclase, an isomorphous mixture of Ab and An molecules, is 

 in respect of the Ab molecules subject to alteration into kaolin 

 (2H 2 O.Al 2 3 .2Si0 2 ). 



Na 2 O.Al 2 3 .6 Si0 2 +2 H 2 = Na 2 0+4 Si 2 + 2 H 2 0.A1 2 3 .2 Si0 2 . 



Two An molecules, with the addition of water and the Si 2 

 liberated from the Ab molecule, give prehnite (H 2 . 2 Ca . Al 2 3 . 

 3 Si 2 ) and kaolin. 



2 (Ca . Al 2 3 . 2 Si 2 ) + 3 H 2 + Si 2 = H 2 . 2 Ca . Al 2 3 . 

 3 Si 2 + 2 H 2 . Al 2 3 . 2 Si 2 . 



In the green veins the orthoclase of the intergrowth, and generally 

 also that of the phenocrysts, is altered to an aggregate of chlorite. 

 Not infrequently the phenocrysts show only a border of chlorite, the 

 interior being altered mainly to epidote. The plagioclase crystals in 

 the veins are often less altered than the orthoclase. They show very 

 fine lamellar twinning on the albite law, with low extinction angles, 

 and may be referred to oligoclase. Bronzite appears to be absent 

 from the veins and segregation-patches. Augite is subordinate in 

 quantity. It rarely shows crystal outlines, and is generally 

 represented by secondary hornblende. Biotite occurs very locally 

 and only in shapeless grains. A little apatite also occurs locally, 

 generally in long needles. Ilmenite is abundant and often shows 

 good crystal outlines, with much production of leucoxene. It also 

 alters occasionally into a semi-translucent, dark-brown substance, 

 slightly doubly refracting, and giving indistinct biaxial effects in 

 convergent light. 



The significance of these veins has been pointed out by both 

 Waller and Teall. To quote the latter: 3 "They are composed of the 

 mother-liquor left after the separation of the more basic compounds." 

 On this view, it may be assumed in the case of the true veins that 

 fissures and crevices, the result of contraction of the cooling mass, 



1 " Ueber den Quarznorit von Penmaenmawr in Wales und seine Schlieren- 

 bildungen": Neues Jahrbuch filr Mineralogie, etc., Abb., pp. Ill et seq., 1905. 



2 Op. cit., p. 115. 3 Op. cit., p. 275. 



