14 Proceedings of the Ro]j<d Society of Victoria. 



in the phonolite from Wolf's Rock, Land's End, England, 

 figured by Rosenbuscli in Fig. 2, Plate XXIII. of his 

 "Microscopic Physiography." Several of the cracks parallel to 

 T, characteristic of sanidine, are conspicuous in each half of the 

 twin and meet on the twinning line at an angle of about 150°. 

 The cleavages parallel to P are only faintly visible, the section 

 not being thin enough to shew them well. The separate halves 

 of the twin extinguish at an angle of 8" with the twinning line 

 and on the opposite side of it, the angle between the two 

 extinctions being thus 16\ The smaller crystals of sanidine, 

 which constitute in reality the mass of the rock, are usually 

 broad and do not present well marked outlines. 



In addition to the all pervading sanidine, a large number of 

 columnar-shaped prisms of a green monoclinic mineral are 

 scattered irregularly over the surface of the rock. Where the 

 section is thinnest the green color is often discharged and the 

 prisms then become brownish or almost colorless. They are 

 practically without action upon polarized light, and, from their 

 evident shelly structure, I am inclined to regard them as 

 augite. Scattered throughout, there are also numerous minute 

 grains of magnetite, usually lounded, but sometimes presenting 

 sharp angles. 



A few other outcrops of rocks with a generally trachytic 

 aspect may be briefly noticed. 



Nareeu. — Amongst al>out a dozen slides prepared from the 

 rocks of this locality, I find that the sanidines are mostly 

 lath-shaped, rarely tabular, and exhibit fluxion structure. The 

 rocks are, however, principally remarkable for the occurrence in 

 them of opal, which appears to fill tolerably large spaces formerly 

 occupied by other minerals, and mainly, I think, by the felspars. 

 Usually it is without action on polai'ized light, but occasionally 

 polarizes vividly in slender fibres radiating from a centre. 

 This phenomenon is explained by Miciiel Levy and Fouqu^ as 

 due to contraction. Included in the opal there appear to be 

 flakes, or rather nests, of tridymite, but on this point I am not 

 certain. In hand specimens the rock is dark grey, and, though 

 undoubtedly weathered, is still hard and tough. It is used 

 locally as a building stone. The specific gravity of the rock is 

 2--44; the percentage of silica is o7"89 and of water 3'49. 

 The other ingredients have not been finally estimated. 



