MacLeod. — On a Hypersthene Andesite. 489 



in foreign andesites, whilst on the other hand the ahiraina, 

 iron, lime, and magnesia are proportionally lower, suggesting 

 that either secondary silica has been iiitroduced into the base 

 or that the rock as a whole is slightly decomposed ; more 

 likely the latter, as I find it very hard in this district to obtain 

 perfectly fresh specimens of andesitic rocks. 



Microscojnc Examination. — Hypersthene and feldspar form 

 the phenocrysts (no augite being visible in the sections I 

 determined), and are set in a base dusted with grains of mag- 

 netite. 



The hypersthenes occur in small crystals up to about 

 1 mm. in length, generally in isolated well-defined crystals, 

 though sometimes in bunches. They are fractured at right 

 angles to the vertical axis, and exhibit also, in some sections, 

 branching irregular cracks. The brachypinacoidal cleavage is 

 noticeable, and in cross-sections sometimes the prismatic. 

 In ordinary light they appear light-brown. The pleochroism 

 is strong, a = red-brown ; b = red-yellow ; c = pale sea-green. 

 The polarisation colours are fairly brilliant, and the extinction 

 straight. 



The feldspars generally occur in plates, accompanied by the 

 lath-shaped type, and vary in size up to about 3 mm. or 4 mm., 

 being much larger than the hypersthenes. By ordinary light 

 they are colourless, but by polarised light they often show 

 polysynthetic twinning, and the extinction angles prove them 

 to be probably andesine, with a little oligoclase. 



The base is largely developed in proportion to the pheno- 

 crysts. By ordinary light it is of a dirty-brown colour (in 

 thin sections a light-drab), and is seen to be dusted with fine 

 grains of magnetite, and by polarised light the base is seen to 

 be micro-crystalline and feldspathic. 



The chief accessory mineral is magnetite, occurring princi- 

 pally in the base, and it is not likely to be a decomposition 

 product of hypersthene phenocrysts, which are fairly fresh 

 and undecomposed. Vitreous inclusions, with gas-pores and 

 negative crystals, occur in the feldspars. Traces of kaolin 

 occur as a decomposition product of these crystals. 



The various constituents follow the normal order of con- 

 solidation, and the rock is a good sample of a hypersthene 

 andesite, the hypersthenes especially being in beautiful 

 crystals. 



Appended (Plate XLVII.) are two diagrammatic water- 

 colour drawings — No. 1, with polariser only; No. 2, with 

 crossed nicols. h = hypersthene ; /= feldspar. 



