IGNEOUS GEOLOGY OF THE BATHGATE AND LINLITHGOW HILLS. 141 



pinacoid. Inclusions of apatite, iron-ores, and devitrified material are usually abundant. 

 The decomposition is interesting and important, and varies in character according as 

 the process begins in the interior or on the exterior of the crystals. In the former case, 

 a grey or pale brown granular material first forms in the centre of the crystal, and 

 renders the transparent augite quite opaque. (See PI. II. fig. 1.) This is usually 

 accompanied by the development or accentuation of the basal lamellar cleavage, which 

 is not always recognisable in fresh augite,* and which, coupled with the simple twinning 

 gives rise to the familiar herring-bone structure. Further decomposition results in the 

 development of viriditic material, either as an irregular felted mass of minute scales, 

 fibres, and granules in the interior of the crystal, or as a regular platy or fibrous 

 aggregate, the fibres being parallel to the basal cleavage and more or less transverse to 

 the vertical axis of the crystal. The process may continue until the whole of the 

 augite has disappeared, its place being taken by a pseudomorph of irregular or of regular 

 structure. The material with a regular arrangement is sometimes a fibrous uralite, 

 with a slightly oblique extinction. At other times it assumes quite a micaceous 

 character, with a distinct cleavage, a marked pleochroism from green to yellowish brown, 

 and a straight extinction. Its bi-refringence, however, is much nearer that of chlorite 

 than of mica, and it ought probably to be referred to the chlorite group. A similar 

 mineral has been described from diabases by various writers, and it is probable that 

 this is also the so-called " delessite " of the earlier investigators. 



Out of the green irregular felted material recognisable minerals frequently develop. 

 Ragged fan-like aggregates of the above "delessite" are found associated with scaly, 

 starry, or spherulitic masses of ordinary prochlorite, nests of minute green needles of 

 actinolite, granules of epidote and tiny crystals of rutile. The production of these 

 minerals is usually accompanied by the deposition of much calcite and quartz, probably 

 largely derived from the augite itself. The quartz occasionally occurs in dihexahedral 

 forms, but its secondary origin is usually recognisable from the nature of its inclusions 

 or the associated minerals. Occasionally in the calcite is found embedded another green 

 micaceous mineral, sharply idiomorphic, with rectangular vertical, and hexagonal trans- 

 verse sections. The pleochroism, except in basal sections, is very strong, from deep 

 green to golden yellow. The cleavage is basal and perfect, and in basal sections part- 

 ings parallel to the sides of the crystal, and meeting at 60°, are frequently to be observed. 

 The bi-refringence is high, giving sometimes the brilliant green of the third order. The 

 greatest absorption is parallel to the cleavage, and basal sections yield a negative 

 uniaxial interference figure. The mineral is probably a slightly chloritised secondary 

 mica, f 



When the decomposition of the augite begins on the exterior of the crystals, the 

 original colour changes to a greenish brown, and the mineral becomes pleochroic and 

 gradually assumes the cleavage and properties of brown common hornblende. The 

 change gradually works inwards, the line of junction in the interior being highly 



* See Harker, Quart. Joum. Geol. Soc, vol. 1., 1894, p. 317. + Flett, Geol. Sur. Mem., sheet 55, p. 128. 



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