BY H. I. JENSEN. 125 



Sp. No.ii. Porphyrite. Loc: High Cliff, Pt.Arkwright. (Plate 

 xii., fig. 12). 



i. This is a darker rock than the one previously described; yet 

 it contains 61-16 % of Si0 2 , that is nearly 2% more than the 

 typical Pt.Arkwright porphyrite. In the field the mass is 

 intersected by numerous red aplitic veins (Sp. No. 121) varying 

 in width from a few inches to several feet. The darker colour 

 of this rock is due to a greater amount of groundmass than in 

 Sp. No.i. This also accounts for the higher Si0 2 percentage. 



ii. Microscopic examination — (l)Texture: («)holocrystalline, (b) 

 porph} 7 ritic with aphanitic base, (c) porphyritic with fine granular, 

 micrographic and cryptographic fabrics. The base varies from 

 cryptocrystalline to microcrystalline. 



(2) Constituents (a) plagioclase, (b) hornblende, (c) chlorite, 

 (d) quartz, (e) apatite. 



(3) The phenocrysts have all a greater refractive index than 

 Canada balsam, and extinction angles in sjanmetrical sections 

 vary from 0° to 26°. The felspars are therefore oligoclase 

 andesine and acid labraclorite. The hornblendes are like those 

 of Sp. No.i. The base presents the same peculiar features as in 

 Sp. No.i., containing cryptographic phenocrysts made up of an 

 intergrowth of quartz and felspar. A considerable amount of 

 chlorite exists in the base. 



(4) Consolidation as in the preceding. 



(5) Name: G-ranophyric Porphyrite. 



Sp. No.121 Aplite. Loc: Pt.Arkwright. (Plate xiii., fig. 13). 



i. This remarkable rock occurs in veins intruding the dark 

 porphyrite at High Cliff, Pt.Arkwright. It is reddish and fine- 

 grained and is at first sight taken to be composed wholly of 

 orthoclase. The presence of quartz may be detected with a lens. 



ii. Microscopic investigation — (1) Texture : (a) crystallinity, 

 noncrystalline; (b) grain-size, variable, mostly fine; (c) fabric, 

 graphic with also micrographic and cryptographic intergrowths. 



(2) Constituents (in order of decreasing amount) — («) ortho- 

 clase, (6) quartz, (c) a small amount of hornblende decomposing 

 to chlorite. 



