20 H. G. Sargent — The Penmaenmawr Intrusions. 



rock in the quarry directly under the summit such sections occur. 

 These show extinctions of about — 12°, and in one or two instances 

 + 7° and + 10°. It may therefore be not unreasonably concluded 

 that the plagioclase in the rock ranges from an acid labradorite near 

 the margin to the oligoclase-andesine series in the central portion. 

 There is no doubt that orthoclase is also present in all the varieties of 

 the rock, but subordinate to plagioclase. 



Pyroxene phenocrysts (both rhombic aud monoclinic) occur in 

 greater quantity in the fine and medium-grained rock varieties (that 

 is to say in the inner portion and up to the centre of the mass) than 

 in the marginal rock. The same remark holds good locally of apatite. 

 A section from the centre (No. 308) shows larger and more abundant 

 crystals than have been observed in any other part. 



Bronzite, whether fresh or in the form of bastite, always assumes 

 idiomorphic outlines, generally in prismatic sections. The terminations 

 have always a ragged and frayed appearance, the result probably of 

 resorption. Augite is generally fresher than bronzite and shows the 

 fine basal striation noted by Teall on both pinacoids. Schaub ! claims 

 that this feature predominates in brownish individuals, turbid through 

 commencing decomposition, and that it may therefore be looked upon 

 as a secondary appearance. 



The proportions of bronzite and augite are fairly equal throughout 

 the fine and medium-grained rock. 



The predominating iron-ore appears to be titaniferous. It occurs 

 in rectangular grains and hexagonal crystals, and often shows beautiful 

 decomposition effects. It frequently contains enclosures of apatite, 

 and has no doubt as a rule followed that mineral in the order of 

 crystallization. It occurs in larger crystals and in greater abundance 

 in the inner part of the mass than at the margin. 



Veins are an important feature in the structure of Penmaenmawr. 

 They are sometimes seen to traverse the whole face of the mountain 

 exposed by quarrying operations (the uppermost 500-600 feet), and 

 they vary in thickness from half an inch up to 6 or 8 inches. When 

 fresh the vein material is of a whitish or light-grey colour, but more 

 frequently, owing to the presence of decomposition-products, it is 

 coloured green of varying shades. 



In addition to the true veins, irregular patches and streaks with 

 similar variations of colour are common. These often display no 

 definite boundary, but merge into the surrounding rock, and seldom 

 extend to any great distance. Judging from their microscopic 

 structure, referred to below, they are of the nature of segregation- 

 patches of the more acid constituents of the rock. When much altered 

 they have frequently a mottled appearance, green and white, owing 

 to the presence of large flakes of quartz in addition to the green 

 decomposition products. Grains of secondary epidote may often be 

 observed in hand-specimens. These veins and segregation patches 

 appear to occur in the western quarries more abundantly in the fresh 

 condition than the green veins, and Mr. T. H. Waller 2 paid special 



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

 bildungen " : Neues Jahrbuch fur Mineralogie, etc., Abh., p. 100, 1905. 



2 Midland Naturalist, 1885, p. 5. 



