GEOLOGY AND ORE DEPOSITS OF SU-AN 15 



g) Basalt. (PL III. figs. 5 and 6.) 



Composition : Essential: Plagioclase, violet augite, 

 olivine, ilmeuite. 

 Accessory: Quartz, barkevikite, apatite, 

 datolite (?), pleonaste. 



Macrotexture : Aphanitic, porphyritic. 



IMicrotcxture : Hypocrystalline, percrystalline. 

 Blackish-gray, compact feldspar-basalt with the idiomorphic mega- 

 phenocryst (2-3 mni) of magnesium-rich olivine. The micrctexture is 

 variable. It is almost crystalline-granular, being a plexus of lath- 

 shaped, multiple-twinned plagioclase and granular, so-called titan- 

 augite. Some specimens have an intorsertal, some a rudely fluidal 

 fabric, the latter being caused by the prismatic habit of both the plagio- 

 clase and the augite. Small patches of colorless base are seen filling 

 up the interstices between the crystal components. The violet, low 

 double-refractive augite is zonally colored, and often shows the typical 

 hour-glass structure. Its form is sometimes prismatic, sometimes ir- 

 regularly tabular and even granular according to the conditions of 

 cooling in the form of either Stielbasalt ov floiu-hasalt. ]\[ega— and 

 microphenocryst of idiomorphic olivine contains pleonaste. The olivine 

 is partially and in some cases completely altered into greenish-yellow 

 fibres, enclosing kernels of hypidiomorphic magnetite and calcite. Ilme- 

 nite is abundant, becoming transparent and brown at its thin margin. 

 In small patches and Scldieren, where augite is scarce and the zeolitized 

 base abundant, the colored component is replaced by a kataphoritic or 

 barkevikitic hornblende ^', and here apatite is found in felt-work. Some- 

 times quartz occurs, fringed with radially arranged, violet augite 

 needles, which become green at the contact with the quartz-eyes (fig. 



1) The brown amphibole is prismatic (110, 010) in habit with imperfect fibrous termina- 

 äons. The extinction c: c = 24° and is too large for barkevikite; strong absorption trans- 

 verse to the principal zone (6 brownish-red with violet tinge, c greenish-yellow, a reddisli- 

 yellow, b > C > a). The hornblende is scarcely distinguishable from the so-called basaltic 

 hornblende. This component is the constant accessory of the Hol-gol basalt. 



