166 Transaction* . 



A few irregular biaxial interference figures were obtained, but no sec- 

 tions were found suitable for definite optical tests. 



Anorthoclase occurs in occasional phenocrysts, especially in the rocks 

 from the upper Pine Hill slopes. The fine indefinite pericline and albite 

 cross-twinning is characteristic. 



Oligoclase occurs in a few crystals. It is recognized by its albite twin- 

 ning, with a low extinction-angle on either side of the twinning-plane. 



In one case sodalite is included in a crystal of sanidine. 



The nepheline phenocrysts are large and fairly plentiful. Character- 

 istic hexagonal cross-sections are not uncommon, but corrosion has been 

 active as a general rule. In some sections no nepheline of the first 

 generation appears ; in others, especially in those from rocks towards 

 Pine Hill, the mineral is comparatively coarse and plentiful. 



Sodalite is plentiful. A few large sharply idiomorphic forms, simulating 

 hexagonal cross-sections of nepheline, are present, but the characteristic 

 sodalite crystal is irregular and flaky. 



Olivine generally is a most plentiful phenocryst. The crystals are 

 large, fresh, and rounded. A " celyphitic " structure, in which pale 

 pinkish-brown augite and iron-ore form a " corona " around the olivine, 

 is marked. Where the olivine has not this corona it shows typically a 

 corrosion border of magnetite dust and a deep fringe of aegirine-augite 

 granules. 



Resorbed amphibole is a constant and characteristic phenocryst. All 

 stages of resorption are exhibited. The cross-sections of even the wholly 

 resorbed mineral show characteristic shape and prismatic angles. The 

 unresorbed mineral shows intense pleochroism, in colours varying from 

 deep brown to golden-yellow. In some sections, particularly those from 

 rocks near the outcrop of hypabyssal trachydolerite, amphibole, next to 

 augite, is the most abundant of the minerals of the first generation. This 

 amphibole has been classed tentatively as barkevicite. 



The occurrence of pyroxene is most commonly in glomeroporphyritic 

 phenocrysts of a pinkish-brown variety of augite. The cleavage is cha- 

 racteristic. Both simple and polysynthetic twins are common. Zonal 

 structure is noticeable. 



This pyroxene was more or less unstable in the original magma, and. 

 though of idiomorphic outline, is almost always edged by a border of 

 aegirine-augite. 



A purplish-violet pleochroic augite is found rarely in well-shaped 

 crystals. The pleochroism is — parallel to c deep purplish-violet, and parallel 

 to a and b greyish-violet. 



A rare deep-green to golden-yellow pleochroic pyroxene is ascribed to 

 aegirine or aegirine-augite. It shows good augitic cleavage on basal 

 sections. Two or three crystals of this mineral are of large size (2 mm. 

 by 3 mm.), and include abundant prisms of apatite and squares of 

 magnetite. They indicate by their irregular boundary that either they 

 themselves have been resolved, or that the aegirine-augite and magnetite 

 are the resorption-products of an earlier mineral. The constant associa- 

 tion of aegirine-augite and magnetite with resorbed amphibole strengthens 

 the supposition that these two minerals are the resorption-products of the 

 amphibole. 



In one case a pale-green augite crystal includes one of olivine. 



Magnetite is infrequent otherwise than as a resorption-product. 



