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Black mica, iron-ores, olivine, apatite, zircon, titanite and other 

 minerals occur in the leucite- and nepheline- bearing rocks, but they 

 call for no special description. 



DESCRIPTION OF THE ROCKS. 



The Wolf rock lies about nine miles south-east of the Land's End, 

 and is covered by the sea at high water. At low water spring tides 

 it measures only 175 feet by 150 feet and stands 17 feet above the 

 sea. At high water it is 2 feet below the level of the sea. The 

 fresh rock consists of a compact grey ground-mass through which 

 porphyritic crystals of a glassy felspar are thickly scattered. Small 

 black specks (nosean) may be recognized with a hand-lens. The rock 

 is markedly fissile in one direction, but breaks irregularly and with 

 difficulty in other directions (ALLPORT). In thin sections the porphy- 

 ritic constituents are seen to comprise sanidine and nosean ; the ground- 

 mass is holo-crystalline and consists of sanidine, nepheline, a few small 

 crystals of nosean and segirine. Iron ores are very scarce. 



A few of the larger crystals of felspar are tabular but the majority 

 are columnar. The columnar crystals give rectangular or lath-shaped 

 sections ; the former extinguish parallel to their edges and the latter 

 give straight extinction or else very low (maximum 4 or 5) extinction 

 angles. As a rule twinning is absent ; a few binary twins may, however, 

 be recognized. Cleavage flakes parallel to M (010) give an extinction of 

 about 5 referred to the trace of the P (001) cleavage. These facts show 

 that the crystals are sanidine, and that the columnar form is due to 

 elongation in the direction of the edge P/M. The extinction in the lath- 

 shaped sections varies from to 5, and the majority give approximately 

 straight extinction. Inclusions of nosean are common in the sanidines. 

 The sanidine of the ground-mass occurs only in the form of columns. 



Nosean occurs in idiomorphic crystals, which very commonly give 

 hexagonal outlines. The smaller crystals are somewhat cloudy or dusty 

 in appearance ; the larger crystals frequently contain the characteristic 

 black inclusions, which are either distributed irregularly or arranged in 

 lines. The central part of a crystal often contains inclusions while 

 the marginal part is free from them, the planes of separation being 

 perfectly sharp and corresponding to the faces of the crystal. 



Nepheline occurs as a constituent of the ground-mass. Cross- 

 sections are hexagonal in outline, often with the angles somewhat rounded, 

 and they of course appear dark in all positions under crossed nicols. 

 Sections parallel to the morphological axis are rectangular and often 

 nearly square. They polarize only in neutral tints and extinguish 

 parallel to the bounding edges. In the fresh rock the nepheline 

 appears to be always water-clear. A careful adjustment of the light 

 is necessary to distinguish the outlines of the mineral, and to make 

 out its precise distribution it is necessary to etch the slide with acid 

 and stain with fuchsin or some other colouring matter. 



