BY H. I. JENSEN. 159 



stituents areplagioclase, segirine-acmite, and glass. A considerable 

 amount of apatite and ilmenite is present. The plagioclase has 

 very low extinction angles, occurs in longish laths, and is 

 probably albite and oligoclase. A little orthoclase (sanidine) is 

 present in grains of irregular shape. The augite (segirine-acmite) 

 is green and slightly pleochroic. It occurs in irregular patches, 

 which ophitically envelop the plagioclase. Plates of ilmenite 

 and idiomorphic magnetite crystals are abundant. Long stout 

 rods of apatite are fairly plentiful. The glass is interstitial. 

 The minerals consolidated in the following order : — magnetite and 

 ilmenite, apatite, plagioclase, augite, orthoclase, glass. This rock 

 probably belongs to the same magma as the main rock of 

 Mt. Cooroy. It is evidently a highly alkaline andesite, or soda- 

 andesite. 



Sp. No.164. Pilotaxitic Andesite. Loc: Dyke in 78-mile railway 

 cutting, between Eumundi and Cooroy. 



i. The hand specimen has the appearance of basic andesite, and 

 •contains large white phenocrysts of secondary calcite and felspar. 



ii. Microscopic examination. — (1) Texture: it is a noncrys- 

 talline, uneven-grained, porphyritic rock, with a pilotaxitic 

 ophitic base. 



(2) The essential minerals are plagioclase and augite. Ilmenite 

 is an important constituent. Dolomite is plentiful. In minute 

 amount apatite and orthoclase are represented. 



(3) The plagioclase occurs in large tabular phenocrysts consist- 

 ing of labradorite containing magnetite inclusions, and occa- 

 sionally calcite as a decomposition product. The plagioclase of 

 the groundmass is andesine, and occurs in lath-shaped microlites. 

 The augite occurs in minute grains in the base. It is of a light 

 greenish variety, allied to acmite. There are also a few corroded 

 phenocrysts of diopside. The ilmenite occurs both in larger plates 

 and minute needle-shaped c^stals. Leucoxene is present in very 

 considerable amount as decomposition product. The dolomite 

 exists in large somewhat regular masses, which appear to be 

 pseudomorphs after a mineral like hypersthene or olivine. They 

 have often a roughly hexagonal section, and each mass is made 



