IGNEOUS GEOLOGY OF THE BATHGATE AND LINLITHGOW HILLS. 139 



surrounding mesostasis. More rarely the intersertal material appears to have exerted 

 a corrosive action upon the felspars. Fresh crystals have the cleavages well marked, 

 and possess inclusions of apatite and iron-ores. Decomposition gives rise to abundant 

 micaceous material mixed with kaolin and calcite, the process in most cases beginning 

 in the interior. Between crossed nicols the crystals are generally striated, showing, 

 however, a comparatively small number of albite, and less frequently a few pericline 

 lamellae. Carlsbad twinning is very generally associated with albite twinning. The 

 great majority of the crystals are imperfectly zoned, giving a progressive or continuous 

 extinction from centre to margin. Untwinned brachypinacoidal sections give angles 

 for the different zones varying from — 30° to +5°, indicating that the crystals change 

 gradually in composition from labradorite to oligoclase, the largest part of the crystal as 

 a rule belonging to labradorite. This is confirmed by observations on suitable macro- 

 pinacoidal sections. 



In some of the grey or red felspathic varieties, however, the crystals of felspar are 

 largely untwinned, or only simply twinned, and rarely zoned. In many long columnar 

 crystals the basal cleavage, probably accentuated by secondary changes, has produced a 

 herring-bone structure, similar to that found in augite, and this, coupled with the simple 

 twinning, has led many observers to refer these crystals to orthoclase.* Neither of 

 these features, however, is confined to monoclinic felspars, and where no more exact 

 determinations haA^e been made such statements as to the presence of orthoclase in 

 diabases should be received with great reservation. Stecher explained the herring- 

 bone structure of the felspars as a micropegmatite of plagioclase and quartz.t Such 

 formations undoubtedly do occur, but usually as intersertal products or corrosion-effects, 

 while columnar felspars with herring-bone structure are, as a rule, free from quartz. So 

 far, the only authenticated occurrence of orthoclase in these rocks is that recorded by 

 Dr Flett from Eastern Fife, where the true nature of the felspar was determined by 

 observations on cleavage flakes.^ Orthoclase in columnar crystals has not yet, however, 

 been found in any of the diabases of the Bathgate Hills. Determinations of the re- 

 fractive index of many untwinned and simply twinned felspars, by means of mixtures of 

 cassia and olive oils of known refractive indices, have shown that such crystals really 

 belong to acid plagioclase. The content in potash which the analyses reveal, must, 

 therefore, be referred to the mesostasis where orthoclase may occur in tiny crystals or 

 in micropegmatitic intergrowth with quartz. 



Pyroxenes are the only ferromagnesian minerals found in the fresh and unattacked 

 rock, and the predominating pyroxene is a pale brown augite which varies much in 

 habit. True ophitic augite in large allotriomorphic plates or polysomatic masses 

 enclosing crystals of felspar is comparatively rare, and occurs locally in the marginal 

 portions of the sills. (See PI. II. fig. 1.) Throughout the dykes, however, and also 

 on the margins of the sills, a hypidiomorphic granular augite with a tendency to ophitic 



* A. Geikie, Trans. Roy. Soc. Edin., vol. xxix., 1880. t Stecher, Tsch. Min. u. Petr. Mitth., vol. ix., 1887. 

 % " The Geology of Eastern Fife," Geol. Sur. Mem., 1902, p. 391. 



